Our picture mats are made to perfectly fit every poster with a white border around its motif. If you choose a 21x30 cm / 8x12 inch poster for example, just pick the picture mat in the same size and it will fit perfectly without covering too much of the image. There are a few exceptions where we think you should not use a picture mat, for example posters with full bleed (picture is printed all the way to the edges of the poster) and for a few posters with a different picture format. Those are clearly indicated in the product descriptions. Our picture mats are acid free, even the core. On top of that they are of museum quality and easy to mount.