# Math Help - multiplying columns and rows

1. ## multiplying columns and rows

My professor drew many pairs of matrices on a sheet of paper. He then asked "Which ones are in illegal positions?"

I believe I already know this but I want to make sure:

You can't have rows on the left, and columns on the right? However, the other way around is fine.

Just want to make sure.

2. Legal:

$
m\;x\;n\;\cdot\;n\;x\;p
$

As long as those 'n's match up, what's the trick?

It's a bit hard to know what you need without a single example.

3. Originally Posted by TheBerkeleyBoss
My professor drew many pairs of matrices on a sheet of paper. He then asked "Which ones are in illegal positions?"

I believe I already know this but I want to make sure:

You can't have rows on the left, and columns on the right? However, the other way around is fine.

Just want to make sure.
That makes no sense. What do you mean by "rows on the left, columns on the right"?

If you are multiplying two matrices, You must have the number of rows in the matrix on the left equal to the number of columns in the matrix on the right. Is that what you mean?