Gravel boards are those prefabricated slabs which are used to ether make small walls, or as a base board on wooden fencing. Some garden fences are made entirely from gravel boards, but this is relatively uncommon. Gravel boards are also used extensively along public highways. There are various different decorative gravel boards, however they are more a quick jobbie rather than an attractive finish.

There are many different gravel sizes, like 20mm Gravel, and when a company is talking about 20mm Gravel (or any other size gravel) they are referring to the average size of each piece of gravel.

Of course by ordering 20mm Gravel you won't get a back in which every piece of gravel measures 20mm exactly, rather the 20mm gravel will have an average size for the whole bag of 20mm gravel pieces.