Coinbase tells me to send from electrum to my private key? A few years ago I sent my BTC from Coinbase to Electrum on windows PC.  That apparently went fine because Electrum showed the correct balance.  Then I received 12 keywords and a hash (starting with 17) which I always thought was my bitcoin key.  I saved these on paper and deleted Electrum, thinking OK I have all I need on paper.
Now  want to transfer my BTC back to Coinbase.  I entered my 12-seeds in Electrum and the balance was right. I hit {Receive] in Coinbase and I get a window giving me the Coinbase(?) address to send my BTC to from Electrum.  But this address is the same as what I thought was my private key (starting with 17).  This is weird especially because 1) How would Coinbase still know this number and 2) Electrum says [Pay it] to (what I thought was) my private key.  What am I missing here.  I don't have many bitcoin but the price is too high for me to make a mistake and get an education.
Tl;dr: what you have is

a seed-phrase for a Bitcoin wallet (where you look after your own cash)
a receiving address associated with a Coinbase account (where you let Coinbase have ownership of your cash in return for an IOU)

You don't have a private key (and you don't need one so long as you keep that seed-phrase written down somewhere safe - not on a computer)


I received 12 keywords

That is almost certainly a seed phrase, also called a recovery-phrase or backup-phrase.
Your master private key is generated from that seed phrase.
In a hierarchical deterministic (HD) wallet, that master private key is used to generate many pairs of private keys and public keys.
For the most common types of Bitcoin transaction, a public key is used to generate a bitcoin address to which money can be sent.

and a hash (starting with 17) which I always thought was my bitcoin key.

A string of around 32, 33 letters and digits starting with a &quot;1&quot; is an old-style of Bitcoin address that is still valid and popular. It is a Bitcoin address, not a private key.
To see some examples look at some reference pages such as

https://en.bitcoin.it/wiki/Invoice_address
https://en.bitcoin.it/wiki/Private_key
https://en.bitcoin.it/wiki/Seed_phrase

Note that the same private key can be shown to you in several different ways (encodings, display-formats).
From the above you should be able to reassure yourself that what you have is a 12-word seed-phrase for your wallet and an  unrelated bitcoin address for your Coinbase account.