Yes it is a 4 sided polygon known as a quadrilateral
The answer is an envelope. It contains a letter.
Envelope is a noun.
It does
it seems to me that the only polygon of your description is a square
A type of polygon is a rhombusial polygon, trysectalnict polygon, and a equilateral polygon.
A triangle (not traingle) is a polygon. It can be a regular polygon but need not be.
The word is "envelope" (which usually holds only one letter).
an arrow is or isn't a polygon?
on the envelope.
This envelope is like the window envelope,but the window opening is uncovered (an uncovered address panel).
The prepositional phrase in the sentence is "on the envelope".
The possessive form for the noun envelope is envelope's.Example: The envelope's flap wasn't sealed.