Before the main spring people relied on big clocks to tell time.  Clocks had to be big because it had to have a spring with a large piece of metal.  The piece of metal had to be bulky in order to be accurate, therefore it couldn’t fit into a small pocket.  A suitable power source was needed to make clocks, but the normal power source couldn’t be made small enough to carry.  The main spring was the answer.  The main spring was invented and in use by the 1400s.  The main spring is a steel coil wound up tightly.  It stores tension when wound around an axle. The axle was also called an arbor.  It slowly turned into a mechanical watch by the 1500s.  The difficulty and precision required to make a pocket watch gave rise to a class of pocket watch makers.  By the 1700s, pocket watches were extremely valuable items.    Being able to tell time without finding a clock tower became a sign of wealth and luxury, which nobles would pay a premium for.  By the 20th century, battery operated clocks/watches became more common and phased out main springs in general, however, the main spring is still used in some fancy watches.  It is my favorite invention because I went to a store to help buy my dad a watch, and I think it is interesting to know about their history.