When a skilled defender looks at dummy, they can often read the key points of the hand. They'll usually know declarer's main plan and, as a result, be focused on how to defend.
A typical example is when you spot a very weak dummy with a singleton and 3 trumps. Declarer will often go for ruffs in dummy, so it falls to the defenders to play trumps. A defender who is clued into that will focus on, for example, getting the lead from the correct side in order to play a trump through.
In this section we'll work through the main categories of dummy. Knowing them helps you form plans in defence and get a better idea of which suit to play, and when.