# What number set is Q (rational) with a line over it?

#### EkajArmstro

I have a question asking if an irrational number belongs to the number set:
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Q

Q normally means rational, but what does that line mean? I assume the irrational number still wouldn't be included in that set?

#### mathman

Forum Staff
Usually a line over a symbol for a set means the complement of the set. In this case the complement of Q is the set of all irrationals (assuming the whole space referred to is the reals).

#### mattpi

It could also mean the closure of the set, in which case (considering it as a subset of the reals, and in the usual topology on $$\displaystyle \mathbb{R}$$) we would have $$\displaystyle \bar{\mathbb{Q}}=\mathbb{R}.$$