# First & Third Angle Projection - idle musing

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I've been reading drawings for the best part of 4 decades and I'm well acquainted with what they are, but always wondered:

why these are so named and also why isn't there a 'Second Angle' projection?

Richard

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I had it explained to me many years ago, maybe around 1980 and I fully understood what was said. Time has however destroyed any details. All I can remember from 1980 is who the prime minister was and I'd much rather forget that :)

When I looked at a drawing recently that clearly said one system I had convinced myself it was the exact opposite. Luckily there was an isometric view otherwise I'd have made it upside down and inside out.

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Conventionally, angle is measured anticlockwise from the horizontal. Consider the horizontal plane and the vertical plane containing y & z axes. The 'first angle' is in the first quadrant, top right. Projections of an object suspended in this quadrant, the plan projected downwards, and the left elevation to the right, are 'first angle'. The 'third angle' is in the lower left quadrant, from 180 to 270 degrees. Projections onto the planes of an object placed there, are upwards and to the right. As far as I can see, you could perfectly well have second or fourth angle projections, but, being unconventional, they would probably be rather confusing.

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These explain it:

and:

Paul

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Thanks Charles, that clears some of the mist. It haddn't occured to me to think in terms of the quadrants in that manner, though I'm aware of the a-c/w convention. However, I still can't see that the result of 1st and 3rd projections as you describe would result in the layout we all use. Taking your scheme for 1st angle so that the projection of the right hand end is projected onto the Y-Z plane on the left, then yes OK, that's 1AP.

But, in the lower left quadrant, you would project the left hand face onto the Y-Z plane to the right - that's still a 1AP - Or are you suggesting the view is always as seen from the positive X direction (for projection onto the Y-Z plane obviously)? That would work, I think.

Rgds Richard

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Thanks Paul, that explains _what_ they are, which I already know. I was more on the 'why and how', but I'll forward the links to a freind who prompted the query in the first place.

Richard

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Equally, it seems one is always looking down. I have not found an exposition of this bit that seems complete either.

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