Etymology
From Thinkmath
Curiosity about words
If it has five (straight) sides, it's called a Pentagon. If it has six sides, it's called Hexagon. And if it has ten sides, it's called a Decagon. All of them are called Polygons. So why aren't three-sided figures called trigons?
Why one word competes out another is often quirky, a combination of historical factors some of which may not even be known. But the "defeated" word rarely goes away entirely, and if one knows where to look, one can often find its descendants.
For example, "trigon" (not a real word) survives in the name of the branch of mathematics that deals with measuring (applying metrics to) Triangles: trigon-o-metry.
A figure with four (straight) sides might have been called a "tetragon" (like Tetrahedron) or even the slightly mixed-up "quadrigon" because quadri- means 'four,' or even a Quadrangle on the model of Triangle, but it isn't (though quadrangle is used for something quite similar). Instead, it is called a Quadrilateral where -lateral refers to sides (as in 'lateral pass'). We don't have bilateral geometric figures, but we do have bilateral agreements, and even unilateral decisions. What do those words really signify?
And while we're on number roots like penta- and hexa-, why aren't October, November, and December the eighth, ninth, and tenth months?
And finally, if the tri- in triple means 'three,' quadru- in quadruple means 'four,' then what does the -ple mean in Multiple? Here are some of its close relatives: the -ble in Double, the -ply in Multiply and also in reply and ply-wood (believe it or not!), the -plic- in duplicate, replicate, and complicate, the -plex in duplex and Complex numbers, and the not-at-all-lookalike word fold.
