In the same way that most people put the horizontal before the vertical for continuous cartesian stuff, but as soon as it's discrete, all hell breaks lose: – S

In the same way that most people put the horizontal before the vertical for continuous cartesian stuff, but as soon as it's discrete, all hell breaks lose: – Some like to think like a matrix then, usually with row, column. – Some like to keep the continuous convention. – Some like to split the difference.