Elasticity of demand is the responsiveness of change in quantity demanded due to change in the price of good.

In real world, we see that increase in the price of a good decreases it's consumption keeping disposable income of consumer same. Increase in the price of chocolate decreases it's consumption.

But if good is a necessity then it's quantity demanded is unchanged due to change in the price of good. In that case, Ed is zero i.e perfectly inelastic. Ed for rice, wheat is inelastic.

Luxuries, like air conditioner, costly furniture, etc have greater than unitary elastic demand. The reason is that change in their prices has a great effect on their demand. Comforts like milk, fans, etc have neither very elastic nor very inelastic demand. Jointly demanded goods, like, car and petrol, pen and ink, camera and film, etc have ordinarily inelastic demand.