The taxonomy of unbalance — static, couple, dynamic, and quasi-static — why couple unbalance is invisible to a single-plane measurement, why two correction planes are sufficient for any rigid rotor, and where the terms static balancing and dynamic balancing come from.

What unbalance is physically, the units and conventions that quantify it, which rotors qualify for single-plane treatment, and how the ISO and API balance tolerance criteria are constructed and compare — the fundamentals behind one of the most common jobs in a rotating equipment repair shop.