The sys­tem fonts Courier, Menlo, and Con­so­las are ex­am­ples of mono­spaced fonts, so named be­cause every char­ac­ter is the same width. When the char­ac­ters vary in width, the font is called pro­por­tional.

The sam­ples above are set at the same point size. But the mono­spaced font (first and third rows) takes up more hor­i­zon­tal space than the pro­por­tional font (sec­ond and fourth rows). The dif­fer­ences are most no­tice­able in char­ac­ters that are nar­row in the pro­por­tional font (like f i j l r t) and the punc­tu­a­tion characters.

Mono­spaced fonts were in­vented to suit the me­chan­i­cal re­quire­ments of type­writ­ers. They were not in­vented to win beauty con­tests. Com­pared to pro­por­tional fonts, mono­spaced fonts are harder to read. And be­cause they take up more hor­i­zon­tal space, you’ll al­ways get fewer words per page with a mono­spaced font.

In stan­dard body text, there are no good rea­sons to use mono­spaced fonts. So don’t. Use pro­por­tional fonts.