# Is there a reason matrix multiplication is defined as row times column and not row times row?

My answer to a question on Quora: Is there a reason matrix multiplication is defined as row times column and not row times row?

Let us look at a basic (and real life) example of matrix multiplication: a matrix made of a single row is multiplied by a matrix made of a single column. This example is calculation of the cost of purchase of some amounts $$g_1,g_2, g_3$$ of some goods (say, apples, bananas, and oranges) at prices $$p_1,p_2, p_3$$ of pounds (of sterling) per kilogram. The answer is well known:

$$\displaystyle{p_1g_1 + p_2 g_2 +p_3g_3 = \sum_{i=1}^3 p_ig_i},$$

which could be conveniently written as a matrix product:

$$\displaystyle{p_1g_1 + p_2 g_2 +p_3g_3 = \left[\begin{array}{ccc} p_1 & p_2 & p_3\end{array}\right]\cdot \left[\begin{array}{c} g_1\\ g_2 \\g_3 \end{array}\right]}.$$

The “row by column” rule of multiplication of matrices conveniently emphasises the fundamental fact that the row vector of fruits and the column vector of prices belong to different vector spaces. For example, they cannot be added — you do not add fruits and prices.

The “row by column rule” is a convenient symbolical expression of a construction know in linear algebra as paring of vector spaces, see Dual pair – Wikipedia. Actually, in this fruit bowl example, it would be natural to take one step further and use upper and lower indices in the notation,

$$p_1g^1 + p_2 g^2 +p_3g^3 = \left[\begin{array}{ccc} p_1 & p_2 & p_3\end{array}\right]\cdot \left[\begin{array}{c} g^1\\ g^2 \\g^3 \end{array}\right].$$

There are deep algebraic reasons for writing matrix products the way this is done, but I do not wish to go into pretty abstract stuff and prefer to limit myself to the most elementary justification known to me.