It is actually 2 sheets of 3/4" plywood glued together to get the 1 1/2" width. The holes are not spaced for anything special. The large holes allow you to hang "stuff" on rafter hangers so it is handy but out of the way as well as running power cords etc. The small holes are meant for 3/4" pegs for mounting additions like a miter saw table. I wish this design was mine but it isn't. Do a search on Trihorse and you will find lots of pictures and youtube videos. I love this design and although it took a long time to build it was worth it.