Odd and Even. Yin and Yang. Substantial and Insubstantial.
There is polarity to everything. We are built with a left side and a right side, and body movement reflects this polarity. Anything we can do with one side, we can do with the other.
We frequently talk about which leg is substantial. Ask yourself, which arm is substantial?
Are the arm and leg on the same side both substantial, or are opposing body portions connected? How does this affect your movement? Each question brings another facet to the endless possibilities of polarity.