Movements must be vertical, not horizontal
The understanding of vertical movements (line) and horizontal movements (line) is vital. Chen Style Taijiquan Practical Method system requires that all movements must be along the vertical line, not the horizontal line.
Vertical line is defined as the lengthwise line; horizontal line is defined as the widthwise line.
When this principle is followed, movements will also comply with the adage of "long overcoming short".
http://practicalmethod.com/2013/12/horizontal-vs-vertical-online-video-trailer/
Source: https://practicalmethod.com/?p=37172