Does the good in question have close substitutes?
This is probably the most important determinant. We were discussing petrol earlier. As a car driver, I think it’s safe to say that there is nothing else that I could put into my car that would make it go! There are no substitutes. So, when the price of petrol rises, I am likely to buy more or less the same amount of petrol, unless I make a concerted effort to use public transport (unlikely, especially if one lived in the countryside) or give up the car (even more unlikely!). Some consumers will make an effort to cut down on petrol, but taking the country as a whole, the fall in the demand for petrol following a given price rise will be relatively very small. Petrol has very inelastic demand. You can probably think of lots of examples of markets where there are many firms all competing with each other selling similar products. Just think how many substitutes there are for CrispyChocs when choosing a chocolate bar in a newsagent. This means that the demand for CrispyChocs is very elastic
