The notation goes back to Frege. It's not easy to google if you don't know what you're looking for, but this seems like a reasonable summary: https://plato.stan

The notation goes back to Frege. It's not easy to google if you don't know what you're looking for, but this seems like a reasonable summary: https://plato.stanford.edu/entries/frege-logic https://plato.stanford.edu/entries/frege-logic You can see that the turnstile symbol |- was already used, and the horizontal line (which was called "Fregescher Schlussstrich", Frege's Conclusion Stroke) in my class, was apparently somehow part of the turnstile itself and only became a separated element in modern notation it seems.