Ascender and descender

They are what give a word its silhouette. Reading is not letter-by-letter recognition, and the bumps above and below the line are a large part of what lets the eye take a familiar word in one go. Take them away — set a word in capitals — and the shape becomes a rectangle, which is one reason all-caps text is slower to read in quantity.

Their length is a design decision with consequences. Long ascenders look elegant and need more line height, or the descenders of one line start colliding with the ascenders of the next. Short ones let you set the lines tightly, which is why interface faces tend to have them, and why a face designed for a screen often looks cramped when it is printed at a large size.