Dr. Edward de Bono pointed out that "parallel" refers to entering the next path, so as to switch between different modes, rather than going all the way along the established path like vertical thinking. Specifically, the differences between parallel thinking and vertical thinking are as follows:

Think vertically: focus on "what". Parallel thinking: focus on "what may become"

Longitudinal thinking: critical thinking. Parallel thinking: constructive thinking.

Vertical thinking: produce an either-or point of view. Parallel thinking: conflicting views are compatible.

Vertical thinking: leading to judgment, questioning and argument. Parallel thinking: listening, understanding, designing and creating.