Synthetic filter discs fit perfectly under your mason jar lid to provide a filtration layer that keeps contamination out of your spawn jars while still allowing gas exchange. These filter discs can be reused many times and cut to different sizes very easily. These filters are 70 mm. If you are looking for filters for wide mouth jars you will need the 90mm filters. If you are looking for regular mouth size filters then you will want the 70mm filters. Synthetic filters discs are different from cellulose filter discs.