Somehow the saddle must to the bike - sounds simple, but it is not! Because the seat should of course be firmly mounted and not wobble or slip. On the other hand, it must be adjustable in height, because only then can a bike be adjusted for different stride lengths. And that's exactly why every bike, whether trekking bike, city bike, road bike or MTB, has a seat post. Rigid seatposts today are usually patent seatposts, here you can learn what makes them so patent..