A perfectly inelastic demand curve is vertical (ie parallel to the price axis).

Price elasticity of demand varies from point to point along a linear normal demand curve.

One of the reasons why petrol (fuel) has a low price elasticity of demand is that it has no close substitutes.

Necessities tend to have a low price elasticity of demand while luxury goods tend to have a high price elasticity of demand.

The producers of a product with an elastic demand will have a strong incentive to reduce the price of their product.