The two macros \setminus and \backslash both produce a backslash in mathmode that is customarily used to denote, well, set differences: $A\setminus B$, $A\backslash B$.

For \backslash there are also a \bigger versions, e.g., \big\backslash, whereas \big\setminus creates an error.

Why is that so? Should we not consider this a bug in \setminus?

  • 3
    No operation symbol grows with \big; why should \setminus behave differently?
    – egreg
    Dec 8, 2015 at 15:45
  • @egreg How about \big/ or \Bigg/? Dec 8, 2015 at 15:47
  • @StevenB.Segletes that's a delimiter (non zero delcode) Dec 8, 2015 at 15:52
  • 1
    OK, TeXnically, the question is answered. Typographically, is it a bad idea to let the set-difference operator scale up?
    – Sebastian
    Dec 8, 2015 at 17:51

1 Answer 1


The difference is in their definition in fontmath.ltx:

\DeclareMathDelimiter{\backslash}    % for double coset G\backslash H


or, in plain.tex,

\def\backslash{\delimiter"26E30F } % for double coset G\backslash H

\mathchardef\setminus="226E % for set difference A\setminus B

Note that if you do texdef -t latex backslash and texdef -t latex setminus you get

macro:->\delimiter "026E30F 


so there's no real difference between the two cases.

There's a substantial definition between a delimiter and a mathchar. The former type will honor \big and friends or \left and \right, the latter won't.

Note that \backslash is defined to be class 0 (ordinary symbol) when used alone, whereas \setminus has class 2 (binary operation symbol); this will affect spacing.

If you need a “bigger \setminus” you can do


but \big\setminus would be like \big+ that's not going to work, because \big needs a delimiter after it, that is, either a character with a nonzero \delcode or a macro expanding to \delimiter<27 bit number>.

The usage of \setminus is very different from / that, traditionally, has no space on either side and so TeX treats it as an ordinary symbol, also assigning it a nonzero \delcode


in LaTeX, and


in plain TeX.

  • 1
    +1 for the workaround, \mathbin{\big\backslash} Dec 8, 2015 at 15:58

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