user38208: From here, we can read that : 

A private key is a 256-bit value, meaning there are approximately
  1.1579e77 possible keys (There are about 1.2288e66 invalid values

But here, we have : 

Nearly every 256-bit number is a valid ECDSA private key.
  Specifically, any 256-bit number from 0x1 to 0xFFFF FFFF FFFF FFFF
  FFFF FFFF FFFF FFFE BAAE DCE6 AF48 A03B BFD2 5E8C D036 4140 is a valid
  private key.

Does "nearly every" mean (1.1579e77-1.2288e66) out of 1.1579e77 ?

user18196: A bitcoin private key is a number greater than or equal to 1 and less than X, where:
X = 0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEBAAEDCE6AF48A03BBFD25E8CD0364141

That's in base 16. In normal base 10 numbers, that's:
X = 115792089237316195423570985008687907852837564279074904382605163141518161494337

Any number outside of that range is an invalid private key. That includes 0 and X, X+1, X+2, ...

38208: <p>From <a href="https://www.reddit.com/r/Bitcoin/comments/1rurll/on_the_subject_of_listing_all_possible_private/" rel="nofollow">here</a>, we can read that : </p>

<blockquote>
  <p>A private key is a 256-bit value, meaning there are approximately
  1.1579e77 possible keys (There are about 1.2288e66 invalid values</p>
</blockquote>

<p>But <a href="https://en.bitcoin.it/wiki/Private_key" rel="nofollow">here</a>, we have : </p>

<blockquote>
  <p>Nearly every 256-bit number is a valid ECDSA private key.
  Specifically, any 256-bit number from 0x1 to 0xFFFF FFFF FFFF FFFF
  FFFF FFFF FFFF FFFE BAAE DCE6 AF48 A03B BFD2 5E8C D036 4140 is a valid
  private key.</p>
</blockquote>

<p>Does "nearly every" mean (1.1579e77-1.2288e66) out of 1.1579e77 ?</p>
 18196: <p>A bitcoin private key is a number greater than or equal to 1 and less than X, where:</p>

<pre><code>X = 0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEBAAEDCE6AF48A03BBFD25E8CD0364141
</code></pre>

<p>That's in base 16. In normal base 10 numbers, that's:</p>

<pre><code>X = 115792089237316195423570985008687907852837564279074904382605163141518161494337
</code></pre>

<p>Any number outside of that range is an invalid private key. That includes 0 and X, X+1, X+2, ...</p>
