There is only a Matrix.
The term vertical or horizontal comes to be when you actually route your matrix for the show -- are the string going up/down or left/right.
Once you have a matrix define as a PP -- you decide what -- and can change anytime -- from vert or horx --- when you assign pixels to your PP.
Joe
Over time --- I will find all of the places that reference Strings and Pixels per string when working with a Matrix and replace the terminology with Rows and Columns.
Joe