Peoples Gas explains its monthly charges are made up of several charges, some of which are a flat rate and some of which are charged by volume, meaning based on how much natural gas is used:
Customer charge (flat rate) $22.06*
Distribution (by volume) 25.96 cents a therm first 50 therms
11.8 cents a therm over 50 therms
Storage charge (by volume) 4.1 cents a therm
Gas charge (by volume) 41.2 cents a therm**
*Rate for customers who buy their gas from Peoples, rather than an alternative supplier
**November rate. Peoples passes along the price of gas at cost.