The Plantin Schoolbook, which comes with the Expression, makes a regular rectangular envelope. The Zoo Balloo and the Celebrations cartridges both have a square envelope, and the Wild Cards Cartridge has a bunch of different shaped envelopes. I don't have very many cartridges, but these are the ones I have that have envelopes. I'm sure a lot of the other cartridges have them. Take a look at the Cartridge Samples for each of the carts at the cricut site
http://www.cricut.com/shopping/productList.aspx?sc=2
That will help you decide which you "have" to have! LOL
Hope That Helps!!!
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Bunny