Different Coil types

Coils come in all different shapes and size variations. The most
common variance is observed by the number of wraps that a
coil will contain, usually anywhere from seven to fourteen. Popular
wrap counts are eight, ten, and twelve. This simply is the
count of how many times the insulated wire is wrapped around
each core. There are different shaped cores on the market as
well, while the most popular is the circular shaped core; however
there is also octagon, square, and oblong shaped cores as
well. Coils will typically be sold in matched pairs. Matched pairs
simply mean that because there is a variance during the wrap
process, two coils will be selected that have complementing
electrical properties, since that will increase the effectiveness
while they work together. How these “effectiveness” qualities are
measured is really up to the manufacturer.