These Tri fold Square cards have a Square aperture

Card size is 140mm x 140mm

The 3rd side is not to fold behind the aperture and stick down as you would with a card used for cross stitching.

It is used to embellish so it is seen through the aperture from the front.

These are 250 gsm white board and are sold in a set of 5 with and without envelopes(envelopes are 100gsm white)

The square is 64mm x 64mm in size and is set 20mm down from the top of the card