You will often see a mattress Spring Unit described as being a '13.5 Gauge' (etc). This term is used often and is rarely explained!

A spring's diameter is measured on a a wire gauge and the strength of the spring is directly related. The lower the number, the firmer the spring.

For instance, a 12 Gauge Spring Unit would be firmer than a 13.5 Gauge Spring Unit. The firmness of the spring then has a direct relationship with the overall firmness of the mattress, but is often refined to give the core of the mattress the strength required. The Spring Unit will then be complimented by fillings of fibre or even latex or memory foam.

Put simply, whenever you see the Gauge of a Spring Unit mentioned, the lower the number, the firmer the spring.