Em

Historically, the em was the width of the uppercase letter 'M' in a given typeface — since 'M' was typically as wide as it was tall, it approximated the point size. In modern CSS and digital typography, em is purely relative to font-size, making it useful for scalable layouts: setting padding to 1 em ensures it always scales proportionally with text. The related 'rem' unit is relative to the root element's font size instead.