# Stretch rule

In classical mechanics, the **stretch rule** (sometimes referred to as **Routh's rule**) states that the moment of inertia of a rigid object is unchanged when the object is stretched parallel to an axis of rotation that is a principal axis, provided that the distribution of mass remains unchanged except in the direction parallel to the axis.[1] This operation leaves cylinders oriented parallel to the axis unchanged in radius.

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This rule can be applied with the parallel axis theorem and the perpendicular axes rule to find moments of inertia for a variety of shapes.