Lesson 3: Length

You'll make extra tricks from long suits  any time your side holds more cards than the other side. So, seven or more cards between you and partner means you may develop extra tricks you need. The technique of length works on both no trump and suit contracts. 

It depends too on how the missing cards are divided between the defenders’ hands. As a general guideline:
• An odd number of cards will divide as evenly as possible. ( e.g. if you have eight cards between you, and there are five, an odd number, out against you,  they will divide 3-2 around 2/3rds of the time, 67.8% to be precise).
• An even number of cards will divide slightly unevenly. (e.g. if you have seven cards between you, and six out against you, they will divide 4-2 more often than 3-3).  

Dummy
♦ 8652
Declarer
♦AK73

In this suit, declarer has two sure tricks – the ♦A and ♦K. If the five missing diamonds are divided 3-2 in the defenders' hands (three cards in one and two in the other), declarer can develop a third trick through length, so win the ♦A and the ♦K and play a third round of the suit. After losing this trick, the last diamond in each hand will produce a third winner.