I have to subscribe to the following and put it out here a bit more to see response. If there is a rule, simplistic in nature it begins with two particle forms in motion having direction interacting with each other.

To simply put the idea down to pictorial representation I will use arrowed lines.

Further to get out of the concept of radical interaction again for simplistic reason of depiction I will put them in opposite direction running parallel to each other.

~…_________________________________________>~

~<__________________________________________... ~

These two lines represent particle form in transient fluid state with infinity source and ending and having direction.