#

The correct backwardly compatible HTML Escape sequence for a number sign
(ASCII - ISO 8859-1 Latin-1)

• In CPP, '#' is the prefix for preprocessor (as if there were the other sort) commands.
• In CPP as GNU extension, inside macros '##' works as an escape that evaluted into a null, with an addition: if used before a "rest" parameter that is empty, it will remove the preceding commas.

  #define foo(a,b...) bar(a,##b)
  foo(a,b)
  foo(a)

  evaluates into

  bar(a,b)
  bar(a)

• Wall character in ADOM.
• Prefix for extended commands in NetHack.
• Comment tag in most text-type configuration files too.
• One of the classic text separators.
• Also known as pound and risuaita.
• Mostly really useless character, which is why it's so damned hard to think of more entries for this list

The # sign (approximated), also known as the sharp can appear amid a song (an accidental), or in the key signature. Any note that falls on the note modified by the sharp will cause any previous accidental or key signature (for that note) to be considered moot and the new, sharped, value to take its place. The value is then raised a half-step harmonically.

For example:

• C becomes C# (which is enharmonically the same as a Db).
• Db becomes a D# (which is the same as Eb)
• E becomes E# (which is the same as F)

And so on.