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We construct market demand curves for products by horizontally summing quantities demanded of a good by each person in the market. So, if there are two people in the market for good $X$, at a certain price, $P^{*}_{X}$, individual 1 will demand $X^{*}_{1}$ and individual 2 will demand $X^{*}_{2}$. We add up $X^{*}_{1}+X^{*}_{2}$, and this gives us the total quantity demanded at the market at $P^{*}_{X}$. 

Market demands will be represented by capital letters from now on, and individual demands by lower case letters. 

This lecture introduces the concept of elasticity. 

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