Personally, I use the acronym HAY and VAX to remember this rule. This means that horizontal lines always correlate with y and vertical lines always correlate with x. A horizontal equation (y=#) is valid, so to say, whereas vertical (x=#) is undefined. It is undefined because there are multiple values of y for the same value of x. You can remember this with the following scenario: A person will always be able to walk in a straight line without getting injuries, but if a person tries to walk up the wall, they will fall and can break their bones, making their body "undefined"
