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(b)

Fig. 89 The planes of projection in "First Angle".

Fig. 89 The planes of projection in "First Angle".

Thus both systems are based on a valid conceptual theory. Both, in fact, are based on the same fundamental theories of descriptive geometry of Descartes, Monge and others. In a way, first angle projection is an evolution as the system, or one very similar, was in use long before the theory was propounded, whereas third angle seems to have been developed by American geometricians from scratch very much later. (It was not standardised, even in the US, until 1935.)

First angle is more or less universal in Europe except for some engineering firms with US parents. Third angle is already standard in the US. As already stated, neither is right or wrong but it is absolutely vital to be consistent right throughout the works or drawing office, and essential to show on every drawing which system is in use. If any deviation from the chosen system is necessary, then direction of view arrows must be used.

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