The proposed Data Item will be the third in the series of Items concerned with the prediction of the drag of rectangular planform cavities for Mach numbers up to 3. ESDU 00006 deals with cavities having closed flow@ and ESDU 00007 is for cavities having open or transitional flow. Those Items are only applicable to cavities without doors@ consequently the proposed Data Item will complete the series with the provision of methods for the prediction of the effect on drag of a pair of open doors typical of weapons bay applications.