Double Thickness with stitching on 2 or 3 sides. These filters will come with or without holes. If a hole is present, it will either be through 1 sheet or go directly through the filter. 1. Measure width and length. 2. If a hole is present: a. Measure across the opening of the hole to determine the diameter. b. Make a note if the hole is through 1 side of the filter or passes straight through both sides of the filter. c. Measure from the closed or stitched points of the filter to the edge of the hole. You should have 3 measurements for the hole position when you’ve finished.