The Anthony Apollo Pool Filters are no longer in production, so they are now being produced by Super-Pro ,Unicel and/ or Filbur. These grids are manufactured at the same high standards, but cost less. The set includes (4) 13.5" X 24" grids and (6) 17.5" X 24" grids. These are not your standard grid! These grids are flat instead of curved. These filter grids are used to hold up the diatomaceous earth or DE powder, which filters the water. To help with efficient filtration and ensure longer lasting grids, it is highly recommended to clean at the beginning and the end of pool season, as well as every three months for longer lasting grids. If the grids aren’t cleaned, the DE powder will not stick to the grid. This will result in clogging of the powder and the water will not be filtered correctly. It is also recommended to backwash every month or as needed to keep your pool functioning properly and looking great.