When people hear "13 postures," they imagine 13 different stances. The 13 Postures (十三势) are actually 13 principles — 8 fundamental energies and 5 directional steps.
The 8 Gates map directly to the I Ching's 8 trigrams: Ward Off (expanding energy), Pull Back (yielding), Press (compact force), Push (downward rooting), Pluck, Split, Elbow, Shoulder.
The 5 Steps map to the 5 Elements: Forward, Backward, Left, Right, and Center — the last being the root from which all movement originates.
"The waist is the commander": all energy flows from the ground through the feet, regulated by the waist, then expressed through the arms.
Tai Chi isn't a collection of moves. It's a unified system with internal logic — a language with grammar, not just vocabulary.