Parallel Axis Theorem
Definition
A formula relating moment of inertia about any axis to the centroidal moment of inertia: I = I_c + Ad², where I_c is centroidal moment of inertia, A is area, and d is distance between parallel axes.
Practical Note
Essential for composite section analysis. The Ad² term is always positive, so I about any non-centroidal axis is always greater than the centroidal I.