Envelope Windows

What is the difference between envelopes with and without windows?

Window envelopes have a clear area on the front to allow the address from inside the letter to be seen, this is to save writing out every address individually.

This table shows the sizes of envelopes and where the window is situated and also its size, there are multiple copies of "DL" and "C4" due to both of these size envelopes having windows in more than one place and size.