What is flammable range?

Petroleum gas and air forms a flammable and explosive mixture. If the percentage of hydrocarbon vapour is too low, the mixture will not ignite and is said to be “too lean”.  If the percentage of hydrocarbon vapor is too high, the corresponding percentage of air will be too  low and the mixture will not support combustion , it is then said to be “too rich”. The mixture will only ignite when the vapour-gas propagation is within a certain range called the flammable range.