ESDU 75006 gives charts and calculation procedures to select the core and faceplate thicknesses of long rectangular sandwich panels for maximum structural efficiency (that is, minimum weight) when loaded on either the long or short sides. Limitations on the faceplate or core thickness can be included. Two alternative design methods are considered, either below or above the limit of proportionality of the faceplate material. The charts assume that the core offers no resistance to loads in the plane of the panel and that the faceplate thickness/core thickness ratio is small enough for the second moment of area of the faceplates to be neglected. The data apply to long panels and will give conservative geometries for finite panels. The error in applying the data to finite panels can be estimated using ESDU 67025.