wire cloth backed microglass fibers. These fibers are much smaller then the cellulose or plasitmeric fibers in cheaper medias. The result is there is a lot more holes or void space in the media so they flow well, have low resistance, can hold a lot more debris and do a far superior job on capturing/holding particles. Think of the microglass fibers as needles vs polly- fibers as toothpicks vs cellulose fibers as pencils in size.
