Scaling uniformly
You might need to maintain the uniformity of geometry as it is being scaled, despite performing nonuniform scaling. The Shift key toggles to uniform scaling operation (from a nonuniform scaling operation) and to nonuniform scaling operation (from a uniform scaling operation).
Note: The Ctrl (Microsoft Windows) or Option (Mac OS X) and 'Shift' keys to allow uniform and non-uniform scaling from the center of the selected geometry.