 The shape of some unusual saddles also makes them physically incompatible with certain seatpost, clamps, or frames, due to bumping into other parts or blocking adjustment or attachment bolts.The most common type of sold-separate clamp has a single horizontal bolt-with-nut which goes behind the top of the seatpost.Tightening this bolt brings together four pieces of metal (two on each side) which have round slots to grab and hold standard-size rails.