⟨THSP⟩
A thread of white, one fifth of an em: the breath the full space overdoes.
The thin space typically measures a fifth of an em — far less than the space bar's blank. It is the breath between initials, around dashes, and wherever the full blank would look like a hole in the line.
Mind its trade: U+2009 breaks the line just like the ordinary space. When the pair must stay together, the right gem is its narrow no-break cousin, U+202F. The entity is  .
Wide: J. R. R. Tolkien (full blanks push the initials apart)
Thin: J. R. R. Tolkien (the thread draws them close)