What's the least signifcant bit of a mantissa system?
The mantissa ranges from
$$
(1.\underbrace{0\cdots 0}_{m \text{ times}})_2 = 1
$$
to
$$
(1.\underbrace{1\cdots 1}_{m \text{ times}})_2 = 2 - (0.\underbrace{0\cdots 0}_{m-1 \text{ times}}1)_2 = 2^{-m}.
$$
For example, $(1.1)_2 = 1.5 = 2 - 1/2$ and $(1.11)_2 = 1.75 = 2 - 1/4$.