I learned about 2.5D, or two and a half dimensions, from my engineering days. Turns out it’s really quite common to make 2.5D things because, for example, it’s easy to drill or mill down into something, and a pain to turn something on it’s side and drill from the other side. Or, as with vacuum forming it’s easy to suck plastic onto a mould, but a pain to get it off if you’ve got any overhangs as you can’t lift it straight up again. Hence the plastic sandpit in our garden is a 2.5D object rather than true 3D. Civilisation in general looks rather 2.5D from the air as we tend to build up, but don’t find overhangs that easy or practical eg the pyramids.
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