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What is the character obtained with they key sequence Esc,Esc called? On my front end it is not displayed, but there's something there messing up my input. It's a bit annoying when I'm after a $\mu$ but hit the wrong key.

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    $\begingroup$ Thanks for the answers guys. I'd just like to note that despite what the help says, [InvisibleComma] does not play nice with ordinary commas. $\endgroup$ – wxffles Feb 7 '12 at 18:51
  • $\begingroup$ Ah, I see what you mean. {a, b, c} is fine, {a\[InvisibleComma] b\[InvisibleComma] c} is fine, but {a\[InvisibleComma] b, c} isn't. Very peculiar... $\endgroup$ – J. M. will be back soon Feb 8 '12 at 0:10
  • $\begingroup$ Thanks a lot! (I'll delete my comments in a few.) $\endgroup$ – J. M. will be back soon May 20 '13 at 0:49
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It's what's called an \[InvisibleComma]. It's useful for those times where you don't want a comma to appear, but you still need it, e.g. "a[[p\[InvisibleComma]q]]" for a matrix entry.

screenshot

The third entry uses an \[InvisibleComma] in between the matrix indices.

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    $\begingroup$ ah. the man himself :) $\endgroup$ – acl Feb 7 '12 at 1:09
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The next time you wonder what a symbol is you can at least partially answer it yourself by entering it in a string wrapped in FullForm:

Mathematica graphics

(this is just before pressing Esc the second time). You would get:

"\[InvisibleComma]"

Which is surely helpful in some way. You can also get the help page for the character by entering it directly into the help search box:

Mathematica graphics

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    $\begingroup$ Or just enter the original input described (Esc,Esc), select the cell, and look at its underlying expression -- on Mac, Shift+Cmd+E. $\endgroup$ – murray Feb 8 '12 at 20:28
  • $\begingroup$ @murray good suggestion, though I think the Box structure is a little confusing to a new user. $\endgroup$ – Mr.Wizard Feb 8 '12 at 21:35

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