Rule 20 of 25 · Chapter IV — Numbers, Lies, and Greens
Club for the shot you hit eight times out of ten, not the one you hit once.
Why this rule exists
Your scoring is set by your typical strike, not your best one, and irons lose distance when they're struck low on the face or slightly thin — which is most of the time for most golfers. Choosing the club that only works with pure contact turns an average strike into a short-sided miss. Take enough club and an average strike still finds the surface.
In practice
Stand on the tee box with a yardage and ask what club gets there with a slightly heavy or slightly low strike, then hit that one. When the number falls between two clubs, take the longer one and swing smoothly rather than the shorter one and swing hard. Notice how often you finish past the hole over a round — if it's almost never, you are under-clubbing.
When it doesn't apply
When long is dead — water, a bunker, or a steep fall-off behind the green — club for the miss you can survive and accept being short. The same applies with a downwind shot into a firm green, where extra club leaves you no chance of holding it.