The plastic layout trays make it easy to create mosaics in a regular grid format and help avoid wiggly lines in your mosaic which you often get when laying tile "freehand". They are intended for glass mosaic tiles but can be used with the 20mm x 20mm unglazed ceramic tiles as well although it's a bit more tricky as the unglazed tiles do not have bevelled edges on the underside and therefore do not sit as snugly in the tray slots. We have trays for both 20mm x 20mm and 10mm x 10mm tiles.