I have recently learned quite a bit on this subject
There are basically two sorts of springs:
1. Linear rate springs -> Same pounds per inch through duration of travel
2. Progressive rate springs -> soft for first few inches and linear for the rest.
Progressive springs are similar to using a normal linear spring, where a helper (or tender) spring, as is used in coilover setups. The progressive rate spring is the equivalent of a linear rate and helper spring.
Progressive springs are wound with a certain coils per inch for most of the travel, and on one end, the coil maker puts more coils per inch. This softens part of the spring. The number of coils needs to be calculated so that the spring 'sags' to the correct ride height under it's stationary corner weight.
I would always recommend talking to a coil spring manufacturer near you, to ensure the car will be safe to drive.
Here's an interesting link:
Spring Link