Springs

Zig Zag Springs (also known as Serpentine springs are used a lot in modern furniture. A minimum of 5 springs should be used under each seating area i.e. chair = 5 springs, 2 seater sofa =10 springs. However the more springs used the firmer the seat.

Double Coned Coil Springs x 9 gauge.
On average a 5" Coil Spring would be used in a chair seat.

Tension Springs are manufactured with plastic covers, and if the cover splits through wear or over stretching the bare wire can cause damage to the cushion covers.

They stretch between 1/2" and 1" when fitted.
The size (length) of the spring refers to the overall size including the hooks.