Full Idea
Frege distinguished between asserting a proposition and expressing it, and he introduced the judgement stroke (a small vertical line, assertion) and the content stroke (a long horizontal line, expression) to represent them.
Gist of Idea
Frege has a judgement stroke (vertical, asserting or judging) and a content stroke (horizontal, expressing)
Source
report of Gottlob Frege (Begriffsschrift [1879]) by Joan Weiner - Frege Ch.3
Book Reference
Weiner,Joan: 'Frege' [OUP 1999], p.31
A Reaction
There are also strokes for conditional and denial.