Janet Tweedy wrote:
[...]
Why doesn't anyone invent a spirit level that's really a flat
square
with the bubble bit in the centre and four radial glassy bits so
you
can see each way at once if you balance the flat square on top of
things

They make them, but they're round, not square, with a little circle
in the middle. Mine's a Hilka, about an inch and a half in diameter:
cheap as chips. Screw-holes provided so you can attach it to
something bigger if you need to.

--
Mike.