Which prefix meaning two is commonly used to denote two sides, as in bilateral?

• A. Bi-
• B. Di-
• C. Quad-
• D. Tri-

Answer: (A)

Two-sided terms in anatomy use the prefix that means two, most commonly bi-. Bilateral literally combines bi- with lateral (side) to express “two sides,” i.e., on both sides of the body. That makes bi- the natural, standard prefix for this concept.

Di- also means two, but it’s used in different contexts (for example, diatomic molecules or digraphs) and is not the usual form for the common term describing two sides. The other prefixes, quad- and tri-, indicate three or four, not two, so they wouldn’t describe two sides.