Question 1142334
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It's just a convention.  
It would have worked if the convention had been that 0 is positive.
It would also have worked if the convention had been that 0 is negative.
It would also have worked if the convention had been that 0 is both positive and
negative.
It also works with the convention that was chosen, that 0 is neither positive nor
negative.
It's better not to be wish-washy, so people picked that convention.

Edwin</pre>