(in dated typography) A space of non-variable width: ⊣ ⊢ equal to the width of a period (full stop) or comma, inserted after ⟨“⟩ or ⟨‘⟩ and before ⟨”⟩ or ⟨’⟩ (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.
Many examples occur on page 204 of the 1837 New Sporting Magazine XIII.
^ Character 'PUNCTUATION SPACE' (U+2008)
Origin of punctuation-space
Used to give consistent presentation of quotation marks irrespective of comma or period placement.