The Cantor set

is constructed iteratively. Starting with the closed unit interval

, the open middle third

is taken out, leaving the two closed intervals

and

. Then the middle thirds of those two intervals are taken out, leaving four intervals of length

, and so on. The Cantor set is the limit (or intersection) of all such sets.
Even though the Cantor set has measure zero and is nowhere dense, the set of sums

where

and

are in

is the whole interval

, because the line

always intersects the set

.