Past exam of the mathematics course of the University of Cambridge / 2015 / iii / Paper 24 / 2 / a / i / Solution

A Suslin line is a dense linear order without endpoints that is order-complete, has the countable chain condition for a linear order, and is not separable in its order topology. Thus every disjoint family of nonempty open intervals is countable, but there is no countable order-dense subset. The completeness condition says that every nonempty bounded-above subset has a supremum. These requirements distinguish a Suslin line from the real line, which has the countable order-dense subset $\mathbb{Q}$.