Euclid — "A figure is that which is contained by any boundary or boundaries."

"In isosceles triangles the angles at the base are equal to one another, and, if the equal straight lines be produced further, the angles under the base will be equal to one another."

"Things which are equal to the same thing are also equal to one another."

"Parallelograms which are on the same base and in the same parallels are equal to one another."

"What do I gain by learning these things?"

"Of trilateral figures, an equilateral triangle is that which has its three sides equal, an isosceles triangle that which has only two of its sides equal, and a scalene triangle that which has its thre…"