Floating point normalised numbers in binary Many text books state that for a binary floating point representation in a computer byte, that if the mantissa is normalised,  then a positive number must start with 01 from the left or a negative number with 10 from the left.This is nothing to do with IEEE standard but a common statement in English A level computing books. (A levels are the school leaving exams in most of the UK, and some other countries.)

They then ask you to spot normalised representations:

1) 100000000

2) 011111111

3) 110000000

In the rule above 1 and  2 would be normalised but not 3. &nbsp;

Why does it have to be 10 and 01? How is this rule derived?
I'm assuming you're talking about IEEE754. Unless I'm missing something, your book is wrong.

Every number in IEEE754 is represented as normalized (starting with a 1), unless your number is subnormal. A number is only subnormal if the mantissa is minimal (all zeroes). Since the first bit is the sign bit, $10$ could be the start of a subnormal number.

For example $10000000000000000000000000000001$ is the subnormal number $-2^{-149}$.