⟨HSP⟩
The smallest visible blank in the catalogue: a hair between two touching gems.
Thinner than thin: the hair space exists to part characters that touch — quotes within quotes, apostrophes leaning on curves.
It is a finishing gem, invisible to the reader and obvious to the compositor. The entity is  ; in well-drawn typefaces, it is the difference between stuck and set.
Touching: “‘Quotes within quotes’”
With hair: “ ‘Quotes within quotes’ ”