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This is a very simple question. I was wondering if anybody might know of a way to generate the symbol for the empty clause in LaTeX (small, character-sized empty square). By ``empty clause" I am referring to the propositional / first-order logic symbol which denotes a clause that is unsatisfiable by default, and is the result of a refutation from a set of unsatisfiable clauses.

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    \fbox{\phantom{x}}? or \begingroup\fboxsep1pt\fbox{\phantom{x}}\endgroup perhaps. Admittedly, this is not a square. \begingroup\fboxsep0pt\fbox{\phantom{\rule{1ex}{1ex}}}\endgroup – user4686 Oct 4 '13 at 21:26
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    Would you be OK with amssymb's \square? – moewe Oct 4 '13 at 21:27
  • Yeah, \square worked somewhat better in terms of size requirements. Thanks! – Jason Oct 4 '13 at 21:29
  • @jfbu \fboxsep=-0.5\fboxrule is possibly better. And \vrule height 1ex width 0pt \kern 1ex for filling the box is more efficient than \phantom{\rule{1ex}{1ex}} – egreg Oct 4 '13 at 21:36
  • You don't have to sign with your name since it automatically appears in the lower right corner of your post. – jub0bs Oct 4 '13 at 21:59
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You have several options available. Some predefined squares or a hand-made one

\newcommand{\Hsquare}{%
  \text{\fboxsep=-.2pt\fbox{\rule{0pt}{1ex}\rule{1ex}{0pt}}}%
}

(requires \usepackage{amsmath}). Here's a table with some of the squares:

enter image description here

In the second column you find what package provides the symbol (next to \Hsquare I could have added amsmath, which is however not a font package). Note that you should say \text{\Squarepipe} if you decide for that symbol.

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