X and Y measurements
 
X and Y measurement is the horizontal and  vertical measurements of a sign.

The X runs horizontal, the Y runs vertical.

In the sign industry and industries that use measurements, the correct terminology would be to say the width (x) before the height (y).

There is also a Z measurement, which is the thickness.  This is particularly important in the sign industry, as many of the products are three dimensional.
 
There are many different thickness of materials available in signage substrates, It's important to keep in mind the Z value of your signage.