An Acute Angle, is that which is less than a right angle, or 90°.
An Obtuse Angle, is that which is greater than a right angle.
Adjacent Angles, are such as have the same vertex, and one common side contained beyond the angular point. The sum of the adjacent angles is always equal to two right angles (13. Eucl. 1.) and therefore, if one of them be acute, the other will be obtuse; and the contrary: whence, if either of them be given, the other is also given, it being the complement of the former to 180°.
Homologous Angles in similar figures are such as retain the same order, reckoning from the first in both figures.
Vertical Angles, are the opposite angles made by two lines cutting or crossing each other. When two lines cut or cross each other, the vertical angles are equal (15 Eucl. 1.)
Alternate Angles, are those cut or obtuse angles made by two lines cutting or crossing each other, and formed by a right line cutting or crossing two parallel lines. Alternate angles are always equal to each other (18. Eucl. 1.)
A rectilineal or right lined Angle, is made by strait lines, to distinguish it from the spherical or curvilineal angle.
Angles of contact. Angles of contact may be considered as true angles, and should be compared with one another, though not with right lined angles as being infinitely smaller.
Angle of elevation, in gunnery, is that which the axis of the hollow cylinder, or barrel of the gun, makes with a horizontal line. See [Elevation].
Angles oblique are those which are greater than right angles.