Maps again. See page 188 for a big map of Salamis, since that is where they are headed. And that is what Dikaiopolis and son are going to see.
The little island between Attica and Salamis, called Psyttaleia, shown also on the opening picture for this lesson,(the little island in the middle of the picture) divided the already narrow sea into further straits in which the Persians, with their big, cumbersome ships were at a big disadvantage, as we'll learn in lesson 15.