The private key is a very long letter/number. But do most people write that down as well?
I don't think so. The only time I have made use of my private key is when am importing my wallet address to MetaMask. What I did was just copy my private keys from my trust wallet, to paste them on MetaMask. Apart from that, I wouldn't have made use of my private keys. Private keys are not something to write down on a piece of paper. The number is very long 
Topic: What is the difference between seed phrase and private key? (Read 351 times)
The private key is a very long letter/number. But do most people write that down as well?
Nope, if you have the seed phrase there is no more need to keep a copy of your private key. Your private key, after all, is derived from your seed phrase. Having the seed phrase under your sole control means you also have the sole control of your private key. If in the event that you specifically need your private key for whatever reason, you could always extract it from your seed phrase using iancoleman's BIP39 tool.
What if you only have the private key but not the seed phrase?
What specifically about it? Take note that your seed phrase could give you your private key but not the other way around. I mean, you cannot derive your seed phrase from your private key.
So if what you have is a private key, you could import it to a wallet and be able to take control of your funds, but only those that are in the address whose private key you are in possession of. Generally, 1 address = 1 private key, but 1 seed phrase = unlimited private keys. So if you have a wallet and lost the seed phrase but have the private key of one of the addresses, you couldn't possibly retrieve those funds that are deposited in addresses other than the one of which you have the private key.
Like Electrum seed phrase, Iancoleman can not be used to generate Electrum wallet private key from its seed phrase because the way the seed phrase is generated on Electrum is not the same as it is generated on BIP39 wallets.
Electrum seed phrases are generated very differently than BIP39 seed phrases, as you say, but this is not why you cannot import them to Ian Coleman's tool. The reason they do not work in Ian Coleman's tool is because the process of turning the seed phrase in to a master private key is (very slightly) different. Ian Coleman's tool doesn't care how the seed phrase was generated, only how it is used. Essentially the only difference is in the salt for PBKDF2 - both concatenate your passphrase (if you are using one) with another word, but BIP39 uses the word "mnemonic" whereas Electrum uses the word "electrum". If you make that very minor change in the code for Ian Coleman, plus an additional change to have to stop checking for the BIP39 checksum (which does not exist in Electrum seed phrases), you will be able to recover your Electrum addresses.-snip-
I would also suggest doing this in this specific case, but do it with a hardware wallet paired with Electrum rather than an Electrum software wallet. 
What if you only have the private key but not the seed phrase?
Since you are asking how to deal with single private keys properly, the wisest decision would be to create a regular wallet like Electrum and then sweep your private key using the "Wallet menu > Private keys > Sweep" option. Sweeping your private key means you will send all the coins that this private key unlocks to a new address that belongs to the new HD wallet you created which is with a regular seed phrase and which is a lot easier to backup and secure. Here is a thorough guide on how to do that: https://bitcoinelectrum.com/sweeping-your-private-keys-into-electrum/ Also, here is the very important guide on How to Safely Download and Verify Electrum.
The second option would be to create a paper wallet with your private key using an offline version of bitaddress https://github.com/pointbiz/bitaddress.org, but I wouldn't recommend doing that because it may result in you losing all your funds especially if you are inexperienced.
The private key is a very long letter/number. But do most people write that down as well?
Just that seed phrase are easy to write down. Private key was created in non deterministic wallets even before seed phrase creation, it can be backup at the time, but I can not recommend writing down private key as I will prefer copying and pasting it. As seed phrase was invented, I can not be comfortable to backup private key, I would prefer seed phrase as it consists of just only 12 to 24 words rather than characters which are hard to backup if compared to seed phrase.you could always extract it from your seed phrase using iancoleman's BIP39 tool.
Yes, but only for BIP39 seed phrase. Like Electrum seed phrase, Iancoleman can not be used to generate Electrum wallet private key from its seed phrase because the way the seed phrase is generated on Electrum is not the same as it is generated on BIP39 wallets.What if you only have the private key but not the seed phrase?
If you have the private key to your coin, you can use it to recover the coin and spend from it. 
What is the difference between seed phrase and private key?
The seed phrase is a 12 word that can be used to get your private key while the private key is derived from your seed phrase. If your seed phrase is multiwallet like trust wallet then you can used it to get your private key from each network like eth, btc. Basically, your seed phrase is like original and the private key is the clone if you put it that way and both can access the wallet.The key that you derive is just a number that can be used in any asymmetric cryptography algorithm granted it is in valid range. So can use a single key as bitcoin, litecoin, ethereum,... key. You don't need to have a special seed.
What is the difference between seed phrase and private key?
The seed phrase is a 12 word that can be used to get your private key while the private key is derived from your seed phrase. If your seed phrase is multiwallet like trust wallet then you can used it to get your private key from each network like eth, btc. Basically, your seed phrase is like original and the private key is the clone if you put it that way and both can access the wallet.What if you only have the private key but not the seed phrase?
You can also import your wallet to a wallet provider that also support private key. Some platform only use seed phrase and some used both. 
The private key is a very long letter/number. But do most people write that down as well?
Nope, if you have the seed phrase there is no more need to keep a copy of your private key. Your private key, after all, is derived from your seed phrase. Having the seed phrase under your sole control means you also have the sole control of your private key. If in the event that you specifically need your private key for whatever reason, you could always extract it from your seed phrase using iancoleman's BIP39 tool.
What if you only have the private key but not the seed phrase?
The private key is a very long letter/number. But do most people write that down as well?
Nope, if you have the seed phrase there is no more need to keep a copy of your private key. Your private key, after all, is derived from your seed phrase. Having the seed phrase under your sole control means you also have the sole control of your private key. If in the event that you specifically need your private key for whatever reason, you could always extract it from your seed phrase using iancoleman's BIP39 tool.
What if you only have the private key but not the seed phrase?
The private key is a very long letter/number. But do most people write that down as well?
Nope, if you have the seed phrase there is no more need to keep a copy of your private key. Your private key, after all, is derived from your seed phrase. Having the seed phrase under your sole control means you also have the sole control of your private key. If in the event that you specifically need your private key for whatever reason, you could always extract it from your seed phrase using iancoleman's BIP39 tool.
The private key is a very long letter/number. But do most people write that down as well?
In terms of saving, they are both important
They aren't, and I would go as far as saying that most users should never interact with their raw private keys unless they really know what they are doing and they have a specific reason for doing so. Trying to extract individual private keys from a wallet for the purpose of back up will likely lead to exposing those private keys and risking the coins contained, or not backing up enough information to fully recover all the coins in the wallet. Most users should just back up their seed phrase and forget about even looking at their private keys.Why call a string made up of letters and numbers (alphanumeric string) a number?
More basically than the answers above: Hexadecimal is a counting system. Hexadecimal numbers are, well, numbers. We simply use the characters A-F to refer to the numbers we more commonly think of as 10-15, because they are characters that are distinct and familiar to everyone. They are not being used as letters, simply as recognizable characters.More precisely, the private key can be any number between 0 and n - 1 inclusive
Why call a string made up of letters and numbers (alphanumeric string) a number? I guess there is a logical explanation behind it.
Any private key is actually an integer between 0 and 2256 and is converted to other formats like WIF. (That's for making the copy-pasting easier and decreasing the chance of mistakes with including a checksum).https://github.com/bitcoinbook/bitcoinbook/blob/develop/ch04.asciidoc
Of course, not any integer between 0 and 2256 can be a valid private key. Due to secp256k1 ECDSA standard, the number of valid private keys is a bit smaller.
Yes. Private key can be any number between 0 and n-1 where n is a constant n = 1.1578 * 1077, slightly less than 2256More precisely, the private key can be any number between 0 and n - 1 inclusive, where n is a constant (n = 1.1578 * 1077, slightly less than 2256) defined as the order of the elliptic curve used in bitcoin (see Elliptic Curve Cryptography Explained). To create such a key, we randomly pick a 256-bit number and check that it is less than n. In programming terms, this is usually achieved by feeding a larger string of random bits, collected from a cryptographically secure source of randomness, into the SHA256 hash algorithm, which will conveniently produce a 256-bit number. If the result is less than n, we have a suitable private key. Otherwise, we simply try again with another random number.
You are not wrong in the way you explained what a private key is because the Bitcoin Wiki refers to private keys as "secret numbers" as well. But I have always wondered why they opted for that terminology? Why call a string made up of letters and numbers (alphanumeric string) a number? I guess there is a logical explanation behind it.
A private key is just a random series of ones and zeroes. You can generate it manually by flipping a coin 256 times. You may have heard of elliptic curve cryptography. A private key is essentially a multiplier that shows how many times we should add the base point G (constant point on an elliptic curve) to itself. The result of this multiplication, that is, elliptic curve scalar multiplication, is some point on an elliptic curve also known as a public key. So, both point G and the point of a public key are on a curve while the private key is not. Public keys aren't random numbers because they are determined by a specific multiplier, but that multiplier is generated by you randomly. Only you know this multiplier, only you can sign a message using this number.
Why call a string made up of letters and numbers (alphanumeric string) a number? I guess there is a logical explanation behind it.
Any private key is actually an integer between 0 and 2256 and is converted to other formats like WIF. (That's for making the copy-pasting easier and decreasing the chance of mistakes with including a checksum).Of course, not any integer between 0 and 2256 can be a valid private key. Due to secp256k1 ECDSA standard, the number of valid private keys is a bit smaller.
A private key is a big random number represented in alphanumeric format.
You are not wrong in the way you explained what a private key is because the Bitcoin Wiki refers to private keys as "secret numbers" as well. But I have always wondered why they opted for that terminology? Why call a string made up of letters and numbers (alphanumeric string) a number? I guess there is a logical explanation behind it.
From above, hosseinimr93 is absolutely right. During the generation of keys and addresses from hierarchical deterministic (HD) wallets, the seed phrase is first generated using entropy and checksum process to generate seed words (the mnemonic or the seed phrase) which is used to generate the seed using key stretching function in which PBKDF2 stretches the seed phrase (or + including the passphrase) using 2048 rounds of hashing with HMAC-SHA512 algorithm. From the seed is the master private key and master chain code are generated. The master private key generates the corresponding master public key (using the normal elliptic curve multiplication process m * G).
The private key, chain code and index number are used to create the child keys.
To know more about seed phrase, keys and addresses, you can read these for it:
https://github.com/bitcoinbook/bitcoinbook/blob/develop/ch05.asciidoc#pubkey
https://github.com/bitcoinbook/bitcoinbook/blob/develop/ch04.asciidoc
The child private key is what you referred to as private key.
There are one way functions which really make it impossible to use private key to generate the seed phrase but the seed phrase can generate the private keys. Know that your seed phrase, seed or your master private key can be used to access all your child private keys which can be used to access your coins and spend it. So know that your child private key can be used to access the coin stored on blockchain with its corresponding address. The reason all private keys (be it child private key or master private key) should be completely remain only known to you, because if known to an attacker, it can be used to steal your coins. So protect your seed phrase, seed, master private key and private keys.
Also know that you master public key can be used to know all the bitcoin connected to your wallet corresponding addresses, it can not be be used to spend but it can deprive privacy, so very good to also protect the master public key for privacy reasons. Only addresses are for the public.
The private key, chain code and index number are used to create the child keys.
To know more about seed phrase, keys and addresses, you can read these for it:
https://github.com/bitcoinbook/bitcoinbook/blob/develop/ch05.asciidoc#pubkey
https://github.com/bitcoinbook/bitcoinbook/blob/develop/ch04.asciidoc
The child private key is what you referred to as private key.
There are one way functions which really make it impossible to use private key to generate the seed phrase but the seed phrase can generate the private keys. Know that your seed phrase, seed or your master private key can be used to access all your child private keys which can be used to access your coins and spend it. So know that your child private key can be used to access the coin stored on blockchain with its corresponding address. The reason all private keys (be it child private key or master private key) should be completely remain only known to you, because if known to an attacker, it can be used to steal your coins. So protect your seed phrase, seed, master private key and private keys.
Also know that you master public key can be used to know all the bitcoin connected to your wallet corresponding addresses, it can not be be used to spend but it can deprive privacy, so very good to also protect the master public key for privacy reasons. Only addresses are for the public.
A seed phrase is a group of 12 - 24 words, while a private key is a combination of 51 or 52 alphanumeric characters.
To be more specific, they are 12, 15, 18, 21 or 24 words. Any shorter than 12 like 9 words and below are not secure. What is the difference between seed phrase and private key?
Both are secret information that you should keep securely because it gives you exclusive access to a specific space inside the blockchain. With these secret keys, you can interact with the blockchain space and move it around, change its size and form. Is there any difference between the two? Yes. A private key is a big random number represented in alphanumeric format. It is used to derive a corresponding public key and a specific address, which you will see in your wallet when clicking the "receive" button. The private key to the given address will be used to send funds to another address. A seed phrase, on the other hand, is a big random number represented as a set of several English words. The software you are using can take this big random number and generate millions of millions of millions of private keys that may be later used to generate millions of millions of millions of addresses which you can use to receive your coins. Each seed phrase always generates the same set of private keys, so you don't need to write them down. Just make sure that you keep your seed in a secure offline place no one unwanted knows about. 
Seed phrase is the parent, private keys are the children.
In terms of saving, they are both important, but saving seed phrase is much better as its readable, can be memorized (not recommended), can be hand written and etc.— than saving private keys, coz its unreadable, too hard to write (composed of random alphanumerics) and there must be so much of them.
Seed phrase are generated using an HD wallet, a hierarchical deterministic wallet.
And private keys can be generated by different methods and might be unsecured in terms of method used.
In terms of saving, they are both important, but saving seed phrase is much better as its readable, can be memorized (not recommended), can be hand written and etc.— than saving private keys, coz its unreadable, too hard to write (composed of random alphanumerics) and there must be so much of them.
Seed phrase are generated using an HD wallet, a hierarchical deterministic wallet.
And private keys can be generated by different methods and might be unsecured in terms of method used.
In basic terms a private keys seed phrase is a representation of the private keys in an easily readable format in which you can back it up. Handling private keys would be difficult for most to write down and store without errors, so the seed or recovery phrase encodes the same information contained in the private key in a more user friendly format for easier recovery of an asset.
Hey Upgrade00, You might want to correct the small error so that OP doesn't get confused when he reads through Jump to: