Vertical combs perforate across the sheet, Horizontals perforate down the sheet. Confusingly, the term 'vertical' refers to the entire vertical row of stamps rather than the direction of the comb. The use of either type is a distinctive indication of the stamp layout of the printsheet since they could not be interchanged in usage.
Vertical versus Horizontal combs can be distinguished by examining any block of stamps but particularly sheets.
This 'pattern' would only be seen once in the entire perforated line but be repeated in the same place for every line.
Conversely, a wandering tooth in the comb (not the line) would be repeated for every stamp along the line of perforations.
The 'line of perforations' examined can of course be horizontal or vertical, thus determining which type of perforator was used for that plate layout.