The story
Used to give consistent presentation of quotation marks irrespective of comma or period placement.
Etymology from Wiktionary, CC BY-SA 4.0.
punctuation space
Today it means: A space of non-variable width: ⊣ ⊢ equal to the width of a period (full stop) or comma, inserted after ⟨“⟩ or ⟨‘⟩ and before ⟨”⟩ or ⟨’⟩ (and sometimes ⟨?⟩ or ⟨!⟩), unless a punctuation mark occurs there. It is Unicode character U+2008. When using X11 input method, the conventional key combination is Compose+Space+Period. Full definition →