Leading

The word is a hot-metal inheritance: strips of lead were laid between lines of type to open them up. What it controls is whether the eye can find the start of the next line without losing its place, and that depends on three things at once — the length of the line, the x-height of the face, and how much room the ascenders and descenders need.

Longer lines need more leading, because the eye has further to travel back. Faces with a large x-height need more, because the lines look heavier and close up faster. A reasonable starting point for body text on screen is 1.5 times the font size, then adjust by looking: if the paragraph reads as a grey block, it is too tight; if the lines float apart into separate objects, it is too loose.