Various sizes of 1mm thick case fan filters. The gaps between the mesh are 1mm squared. The 120, 92, 80, and 40mm filters use 10, 6, 4, and 1 gram(s) of material respectively. Could be disposable, or just as easily washed and reused! Simply install between the case and the desired fan. I personally recommend use with high static pressure fans (as opposed to high airflow fans) and with air intake fans (as opposed to ventilation fans).