An ascender is an upward part of a letterform (often a vertical [stroke](/glossary/stroke)) that extends above the [x-height](/glossary/x_height) and usually above the [cap height](/glossary/cap_height), such as the stem of a lowercase b or d. A descender goes the other way: It’s a downward vertical stroke that extends below the [baseline](/glossary/baseline), such as the stem of a lowercase g or p.
Because very pronounced ascenders or descenders add to a typeface’s overall height, they effectively reduce the x-height. Similarly, if a typeface is said to have a high x-height, it’s usually safe to assume that it therefore has short ascenders and descenders.
In paragraph text, the interplay between the descenders of one line with the ascenders of the line below can be a good indicator of an appropriate [line height](/glossary/line_height_leading) setting. If they touch, the line height is too tight.