Why is a decagon called a decagon?

Answer 1

Mathematics tends to use Greek words, or combination of words, to describe geometric shapes.

Decagon is made up of two parts, based on Greek words:

1. deca- (a transliteration of δέκα) which means "ten"
2. -gon (coming from γωνία, transliterated as "gonia") which means corner or "angle".

So a "decagon" is literally a "ten cornered" or "ten angled" shape.

A "polygon" is also made up of two parts, based on Greek words:

1. poly- (coming from πολλοί, transliterated as "polloi") which means "many"
2. -gon as above meaning corner or "angle".

So a "polygon" is literally a "many cornered" or "many angled" shape.

[A transliteration is re-writing the letter of the original language in letter of our language with a similar sound.]

As an aside, the three-sided and four-sided shapes do not follow this trend, for if they did:

• a triangle would surely be called a triagon (though "triangle" is a translation of the Greek);
• a quadrilateral would surely be called a tesseragon.

since tria is the Greek for 3 and tessera the Greek for 4.