These Tri fold DL cards have a circle aperture

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 circle is 50mm in size and is set 25mm down from the top of the card.