The lengths and spacings of twins in YBa2Cu3O7−&dgr;thin films deposited onto MgO substrates have been measured by transmission electron microscopy as a function of film thicknesst, fortranging from 50 to 1400 nm. The twin length is linear int, while the twin spacing follows at1/2dependence. This form for the twin spacing is consistent with the prediction of a simple free energy expression for the twinning transformation.