A needle board is simply a piece of metal, usually aluminum that has many holes for holding felting needles. The boards came in many sizes and hole patterns. They can be covered with hard plastic on the needle entry side, with rubber on the needle exit side, or not covered at all. Most boards have 15 ga holes in which the needles fit. Some boards have large holes that hold metal or plastic inserts. The insert then has a 15 ga hole that holds the needle. Using inserts extends the life of the board, but inserts limit the density of needles in a board. Boards on some finish looms can be very dense with needles, while preneedle or tacker boards may have a low density of needles. Some manufacturers make preneedle boards that have varying densities of needles with low density at the entrance side and higher density at the exit. Needle boards are either bolted onto the loom or held in place by pneumatic clamps.