Sounds like a good brain‑toss. Symmetry in a parry is basically a game of mirrors—each side tries to keep the angles equal, so the vector of the incoming blow and the counter‑strike line up. If you plot the force vectors, you’ll see they’re just rotations of each other around a pivot point. The math? It’s a simple 2‑D rotation matrix: [cosθ –sinθ; sinθ cosθ] times the incoming vector gives you the outgoing. If you keep the pivot at the guard, the angles are 180‑minus the attack angle, so the sum is 180 degrees. Think of it like a line of sight: you rotate your blade by exactly the same amount you’re turning your body, so you’re always mirrored. So yeah, the “hidden math” is just rotational symmetry and some basic trigonometry. Ready to crunch numbers or just want to feel the heat of a good parry?