In original usage, an isosceles triangle was understood to have the third side unequal to the other two, that is, an isosceles triangle has exactly two equal sides. An equilateral triangle is, however, a special case of an isosceles triangle, having all three sided equal.
- c 300 B.C.E. Euclid Elements, Book I, Definition 20, English translation,
- Of trilateral figures, an equilateral triangle is that which has its three sides equal, an isosceles triangle that which has two of its sides alone equal, and a scalene triangle that which has its three sides unequal. (Τῶν δὲ τριπλεύρων σχημάτων ἰσόπλευρον μὲν τρίγωνόν ἐστι τὸ τὰς τρεῖς ἴσας ἔχον πλευράς, ἰσοσκελὲς δὲ τὸ τὰς δύο μόνας ἴσας ἔχον πλευράς, σκαληνὸν δὲ τὸ τὰς τρεῖς ἀνίσους ἔχον πλευράς.)
- 1551 Recorde, Pathways to Knowledge,
- There is also an other distinction of the names of triangles, according to their sides, whiche other be all equal..other els two sydes bee equall and the thyrd vnequall, which the Greekes call Isosceles ...
- 1570 Billingsley, Euclid Book 1, Definition xxv,
- Isosceles is a triangle, which hath onely two sides equall.
- 1571 Digges, Pantometria I,
- Isoscheles is such a Triangle as hath onely two sides like, the thirde being vnequall, and that is the Base.