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Given the following plain TeX snippet and it's output:

hallo werld

{\catcode`\ =12
hallo wereld}

{\tt
hallo wereld}

{\tt\catcode`\ =12
hallo wereld}

Screenshot of pdfTeX output on given example

The command \catcode`\ =12 changes the category of the space character to "Other". This will make TeX display it.

The space glyph in a typewriter typeface looks familiar. However, the glyph shown in the default typeface I don't recognize.

Out of curiosity, does it have a name? And what is it used for?

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As defined in Unicode, the "open box" glyph in the typewriter corresponds to fileformat.info/info/unicode/char/2423/index.htm –  Pindatjuh Aug 13 '12 at 17:54
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2 Answers

up vote 7 down vote accepted

TeX doesn't use a real character from the current font for spaces, so Knuth decided to fill also the slot usually reserved for it with a rarely used glyph: the slash for producing the "Polish suppressed ell" ł. Here are the relevant lines in plain.tex

\def\l{\char32l}
\def\L{\leavevmode\setbox0\hbox{L}\hbox to\wd0{\hss\char32L}}

This is also supplemented by a kerning entry in the TFM file:

(LIGTABLE
   (LABEL O 40)
   (KRN C l R -0.277779)
   (KRN C L R -0.319446)
   (STOP)
   ...

Since the character has width 0.277779pt, when it comes before a lowercase "l", its width is compensated for.

In the cmtt10 font (and the other typewriter type fonts), the slot is filled with a "visible space".

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It looks like the slash on U+2422 (BLANK SYMBOL): ␢. Since LaTeX has been known to overlay slashes on regular characters (e.g., the symbol produced by \hbar), I bet the underlying b was erased or omitted.

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