Taijiquan classics mention the five directions, but it seems to me in common practice in so-called internal arts, people tend toward these kinds of directions:
1. taijiquan – backward then forward, yield then issue
2. bagua – forward on a diagonal in a circular way
3. xingyi – forward as linearly as possible – the smallest possible advantageous angle
4. aikido – circular around uke or circular on a central pivot such that uke goes around tori
Not to stereotype these arts too much, but that seems to be the pattern to me so far in actual practice by most people. I think most arts’ ideal would be any combination of these directions, plus up and down, at any given moment depending on circumstances. If these arts are meant to have these directional patterns, it’d be nice to know explicity if those are self-imposed limitations by the style as it seems to be in xingyi and aikido, or if there is an apparent mismatch in theory and practice as seems to be the case in taijiquan. That way we can train in the style’s given framework but be very aware we are doing so in order to not forget about all possible other directions. That way we can take advantage of any slight opportunity assuming this observation holds true. Ideally we’d use any of those directions depending on the circumstance. That is where taijiquan theory on five directions and baguazhang theory based on constant change seems to be the same useful concepts (at least when it comes to the underlying or overlaying philosophical components) even if people aren’t following them this way in most practice. Others seem to have decided combining “bagua stepping” with taijiquan makes sense for them perhaps since “yielding” doesn’t have to imply backwards movement.