Must see the horizontal; mustn’t see the vertical
In Practical Method, horizontal moves must be seen. Feeling and thinking you have horizontal size is not enough. It must be real!
Verical moves, however, are the opposite. By nature, vertical moves don’t create horizontal size and therefore no visible to the human eye. Whenever vertical moves are seen, it indicates there is horizontal movement involved and therefore it is an error.
In both cases, a point of reference with a physical object is the best way.
Source: https://practicalmethod.com/?p=75149