Triangular stamps with the longest side towards eachother, appear like square stamps.
Also diamond shaped stamps (parallellograms) fit to eachother quite nicely too.
But there are stamps that trapezoid for example, aren't that easy to combine on a sheet.
And jet ... they exist as well.
The list of examples however is not as long as with the other shapes. For obvious reasons of course.
Here some examples :
Monaco was the first to issue a trapezoid stamp
Also Malaysia got a trapezoid stamp in 1967
Is it a coincidence or not, but the countries that have issued trapezoid stamps, all start with an M...