A quantity similar to a moment of inertia and appearing in the inertia matrix. Suppose a rigid body consists of n particles P₁, P₂,…, P_n, where P_i has mass m_i and position vector r_i = (x_i,y_i,z_i). Then

For a continuous distribution of mass, the products of inertia are defined by corresponding integrals.

When the coordinate planes are planes of symmetry of the rigid body, the products of inertia are zero. In fact, by the spectral theorem, through any fixed point there is a set of three perpendicular axes, called the principle axes, such that the corresponding products of inertia are all zero.