Fig. 40. Right Angle.
Fig. 41. Acute Anglo.
Fig. 42. Obtuse Angle.
If one straight line meets another (extended if necessary), Fig. 40, so that the two angles thus formed are equal, the lines are said to be perpendicular to each other and the angles formed are called right angles.
An acute angle is less than a right angle, Fig. 41.
An obtuse angle is greater than a right angle, Fig. 42.
A surface is produced by the motion of a line; it has two dimensions - length and breadth.