There are basically two types of clock mainsprings. They are hole end and loop end. The hole end is used inside a barrel and has a "D" or oval shaped hole at one end of the mainspring. This allows the mainspring to be attached to a fixed point such as a barrel hook. The loop end has either an oval or round closed end to allow a post or other fixed movement stem to be used as the "anchor" of the mainspring. On smaller loop end mainsprings, the loop end can be open.
Dealing with clock mainsprings is VERY dangerous. They are full of potential energy just waiting to be released. Always wear eye protection (Safety Glasses) and leather gloves.