Paper and come with mailing envelopes made out of superior paper. The invitations are post card style (non-folding) and come in a colorful design that your guests will absolutely love. There are 15 cards in total with 15 accompanying white envelopes all measuring 5 x 7 inches (A7 format). The birthday invitations for kids are single-sided and can be written on with pencil or pen without smearing. Not only do they come in a vibrant multicolor design but they are thick making them more durable. Copyrighted design registered by Papery Pop.