user1976: My question is about the difference between compressed keys and addresses, and i know it was addressed in another question but my question is less about the theory and more about how the keys are used practically in Bitcoin.
So heres what I did: I used dumpprivkey for an address 183Tw2TqXKkbk5ZeocTYwmxg8x46ADXb6c in the .7.2 client, and got L5DTi7rgqsPg7Vq1vTU2dP2HWMEsNM5w5JCqEDXXXXXXXXXXXX. When i pasted this into bitaddress.org's wallet details script it returned several things. First, it showed a different address 1FKswUuFsjWKyr4ZPSe3vEpp1iCXXChBZm, but it also showed the address above and said it was the compressed version. I sent like 0.0001 to each of the 2 addresses and my satoshi client only shows a transaction recieved from the 183 (compressed) address.
Can someone please explain in practical terms what the difference is and why they both even exist in Bitcoin? Any help is appreciated.

user33453: "Still, shouldn't a post-0.6.0 client recognize both the send to the compressed address, and the send to the uncompressed address, because it has the private key sufficient to spend either? – Quizzical Nov 27 '13 at 2:16"
You have a private key? Great! Open bitaddress.org(offline) and scroll to wallet details tab, then enter the private key. You'll find the compressed and uncompressed addresses in the right and left side respectively. Scroll down you'll find the compressed and uncompressed private keys there.
Yes! Private keys when denoted in base 58 check format have different values in compressed and uncompressed form. This was done to ensure that the clients don't get confused as to which address to map (compressed or uncompressed).
The point being: WIF format for every hex private key are different in compressed and uncompressed form.
Hope this answer helps.

user15552: Though Bitcoin initially took only one; the uncompressed public keys, if it then changed to not allow the use of uncompressed keys (and only allowed the use of compressed keys) those people with wallet addresses that correlated to a private key's uncompressed keys would have been unable to access their funds.  
This is because a Public Key:

02E9A095A6A5790BC82FEADE07EE6FC77B05BC4DE7F3790C36D2ECC886D9EC0AC0 (compressed)

Has an address of

1MEqCrJfxPAh2uCcGmAV2Bgkj6qD69XgfF

While the uncompressed version of that public key:

04E9A095A6A5790BC82FEADE07EE6FC77B05BC4DE7F3790C36D2ECC886D9EC0AC0E44402759C51ED0D3BA2F53E749B30A6D1772F0DAE1E3F465E8C8828DF899FE2 (uncompressed)

Has an address of 

1JGTdegLcK8N9mqwhXmGjeUgbQNugii3rm

If Bitcoin stopped accepting your uncompressed version, you'd have to find a compressed public key that would result in your same address to continue to access your funds.  Doing that is just as difficult and unrealistic as trying to crack the bitcoin algorithm.

user208: The reason they exist both, is because Satoshi didn't know about compressed public keys, and it was only recently discovered that they would be possible to use without compatibility problems (support only exists as of version 0.6 of the reference client).
The advantage is clear: their public keys are smaller, resulting in smaller transactions on the network, saving block chain size for everyone.
Update: inside SegWit spends, only compressed keys can be used.

1976: <p>My question is about the difference between compressed keys and addresses, and i know it was addressed in another question but my question is less about the theory and more about how the keys are used practically in Bitcoin.</p>
<p>So heres what I did: I used <code>dumpprivkey</code> for an address <code>183Tw2TqXKkbk5ZeocTYwmxg8x46ADXb6c</code> in the .7.2 client, and got <code>L5DTi7rgqsPg7Vq1vTU2dP2HWMEsNM5w5JCqEDXXXXXXXXXXXX</code>. When i pasted this into <a href="https://www.bitaddress.org" rel="nofollow noreferrer">bitaddress.org's</a> wallet details script it returned several things. First, it showed a different address <code>1FKswUuFsjWKyr4ZPSe3vEpp1iCXXChBZm</code>, but it also showed the address above and said it was the compressed version. I sent like 0.0001 to each of the 2 addresses and my satoshi client only shows a transaction recieved from the 183 (compressed) address.</p>
<p>Can someone please explain in practical terms what the difference is and why they both even exist in Bitcoin? Any help is appreciated.</p>
 33453: <p>"Still, shouldn't a post-0.6.0 client recognize both the send to the compressed address, and the send to the uncompressed address, because it has the private key sufficient to spend either? – Quizzical Nov 27 '13 at 2:16"</p>

<p>You have a private key? Great! Open bitaddress.org(offline) and scroll to wallet details tab, then enter the private key. You'll find the compressed and uncompressed addresses in the right and left side respectively. Scroll down you'll find the compressed and uncompressed private keys there.
Yes! Private keys when denoted in base 58 check format have different values in compressed and uncompressed form. This was done to ensure that the clients don't get confused as to which address to map (compressed or uncompressed).</p>

<p>The point being: WIF format for every hex private key are different in compressed and uncompressed form.
Hope this answer helps.</p>
 15552: <p>Though Bitcoin initially took only one; the uncompressed public keys, if it then changed to not allow the use of uncompressed keys (and only allowed the use of compressed keys) those people with wallet addresses that correlated to a private key's uncompressed keys would have been unable to access their funds.  </p>

<p>This is because a Public Key:</p>

<blockquote>
  <p>02E9A095A6A5790BC82FEADE07EE6FC77B05BC4DE7F3790C36D2ECC886D9EC0AC0 (compressed)</p>
</blockquote>

<p>Has an address of</p>

<blockquote>
  <p>1MEqCrJfxPAh2uCcGmAV2Bgkj6qD69XgfF</p>
</blockquote>

<p>While the uncompressed version of that public key:</p>

<blockquote>
  <p>04E9A095A6A5790BC82FEADE07EE6FC77B05BC4DE7F3790C36D2ECC886D9EC0AC0E44402759C51ED0D3BA2F53E749B30A6D1772F0DAE1E3F465E8C8828DF899FE2 (uncompressed)</p>
</blockquote>

<p>Has an address of </p>

<blockquote>
  <p>1JGTdegLcK8N9mqwhXmGjeUgbQNugii3rm</p>
</blockquote>

<p>If Bitcoin stopped accepting your uncompressed version, you'd have to find a compressed public key that would result in your same address to continue to access your funds.  Doing that is just as difficult and unrealistic as trying to crack the bitcoin algorithm.</p>
 208: <p>The reason they exist both, is because Satoshi didn't know about compressed public keys, and it was only recently discovered that they would be possible to use without compatibility problems (support only exists as of version 0.6 of the reference client).</p>

<p>The advantage is clear: their public keys are smaller, resulting in smaller transactions on the network, saving block chain size for everyone.</p>

<p>Update: inside SegWit spends, only compressed keys can be used.</p>
