Euclid's Elements - Book III, Proposition 31
An angle inscribed in (see Definition 8) a semicircle is a right angle.
An angle inscribed in a segment larger than a semicircle is acute.
An angle inscribed in a segment smaller than a semicircle is obtuse.
Furthermore, the angle of (see Definition 7) a segment larger than a semicircle is obtuse, and the angle of a segment smaller than a semicircle is acute.