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A figure space is a typographic unit equal to the size of a single typographic figure (numeral or letter), minus leading. Its size can fluctuate somewhat depending on which font is being used. This is the preferred space to use in numbers. It has the same width as a digit and keeps the number together for the purpose of line breaking.
In Unicode it is assigned U+2007 FIGURE SPACE (HTML
WORD JOINER should be used if the intent is to merely prevent a line break