My Part of this discussion concerns development for reliability—the part after the original design and before production starts. But this stage cannot be divorced from the other two. All designs evolve from past experience of one sort or another or analogies. So new designs are evolved which use bricks or assemblies or mechanisms which have been used previously, even though perhaps in another way; these pieces have therefore been tested and there is practical experience of their behaviour. This applied to the original design. It is certain, however, that this will require modification, because in the confined space and with the extreme technical requirements demanded in the aircraft industry, any initial design will require modification—if not the scheme has been too pedestrian in concept and will be out-classed by competitive ideas evolved with more daring. Thus once again the designer needs development knowledge.