Another key idea in chess is the loose piece. A loose piece is simply a piece that has no protection. It is common for players to leave pieces unprotected here and there; as long as they aren’t being attacked, they look safe enough. But loose pieces make perfect targets for the double threats described a moment ago. Suppose your queen performs a fork, attacking your opponent’s king and one of his rooks at the same time. He moves his king. Now you can use your queen to take his rook—if it is unprotected. But if the rook is guarded you won’t be able to take it because the cost will be too high: your queen will be captured afterwards.
We can turn this point into advice for practical play. You want to be aware of loose pieces on the board at all times. Any piece your opponent has left unguarded is a possible target for a tactical strike; any piece of yours that is left unguarded is a vulnerability. Indeed, you want to not only notice loose enemy pieces but also look for ways to create them. We will see countless examples in the studies to come. ("Loose pieces" also can be defined to include enemy pieces that are underdefended: attacked once and defended once by a fellow piece. As we shall see, pieces in that condition sometimes can make targets just as good as pieces with no protection at all.)
The great chess writer Cecil Purdy stated the point as a rule: "Never leave or place a piece loose without first looking for a possible fork or pin, and never see an enemy piece loose without doing the same." Do you follow this advice already? Many inexperienced players don't. When they put a piece onto a new square, they mostly just check to make sure it won't get taken there. Purdy's advice is different. It is to ask whether your piece has protection on its new square; and if it doesn't, to ask carefully whether a fork or pin or other tactic might be launched against it. You may not yet understand quite what it means to look for forks or pins, but you will soon; and then following Purdy's counsel will save you many sorrows.