Naming the Angles of Isosceles Triangles
A lettering convention for triangle vertices, with a note that spherical angles are smaller than straight ones
Two isosceles triangles are drawn, one with its apex pointing down and one pointing up, each with its three corners lettered a, b and c. The accompanying notes set out a convention for referring to an angle by placing its vertex letter between the other two, so that "angle abc" means the angle at b. A further rule states that naming only two sides implies the third, unnamed side is the base, while a separate maxim observes that spherical angles are smaller than straight ones.
On this page
Spherical angles are smaller than straight angles
An isolated maxim at the head of the page states that the spherical angles are less than the straight ones, contrasting an angle measured on a curved surface with a rectilinear one.
Naming an angle by its middle letter
When naming an angle, its own vertex letter is set between the other two, so that saying "angle abc" designates the angle at b. The rule is attached to the inverted isosceles triangle lettered a, b, c.
Two named sides imply the base
If one names the sides "b a" and "a c" without also naming "c b", the side left unnamed is understood to be the base of the triangle. The note accompanies the second, apex-up isosceles triangle a, b, c.