Leopold Clay Tile may be the most popular gravel support underdrain.  These types of underdrains are made of terracotta tile and have patterned holes for water flow through the filter, as well as, backwash flow.  Clay tile underdrains, again, will have gravel support in layers.  These layers include:

 

- 3/4" X 1/2"

- 1/2" X 1/4"

- 1/4" X 1/8"

- 1/8" X #16

- Sand 

- Anthracite