# Numerical analysis:: meaning of “half a unit in the nth place”

What exactly does it mean by "half a unit in the nth place"? I am reading numerical analysis and this term is popping out very frequent in the first chapter itself,my first encounter with this is while reading the rounding-off error rules.

From few solved examples I can understand (or probably guess) it mean $5$ but what I still don't understand why such a abstruse term ?

Thanks,

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