Convergence
This one amounts to the same thing as en prise, but it's trickier to recognize. Convergence refers to a situation where there are two pieces (or pawns) threatening to capture a piece or pawn that has only one defender. By extension, there can be more than two attackers and more than one defender, but there are always more attackers than defenders in convergence.
The bishop and queen converge on f7, which is only defended by the king.
It's Defended
The very fact that the object of the converging attack is defended is what throws off the inexperienced player. Yes, it is defended, but the defense is insufficient.
Being able to look at positions that have not happened yet and compare them to the position that is actually in front of you is an essential skill in chess. A good drill is to set up any position that contains a convergence and try to figure out all the captures in your head. Then play them out, and see how close you came to the positions that actually emerge.
In order to appreciate the power of convergence, you will have to learn to look ahead at least two moves. If the convergence contains three attackers and two defenders, you will need to look ahead at least three moves. But since those moves are all captures, and the captures all take place on the same square, you should be able to manage it.
White has three attackers on the c5-knight, while Black has only two defenders. The play goes 1. Bxc5 Nxc5 2. Rxc5 Bxc5 3. Qxc5.
White winds up a piece ahead.
Unit Value
Another tricky part of convergence is when the attacking units, or even just one of the attacking units, are more valuable than the defending units. In those cases, a simple count of attackers and defenders is not sufficient.
Again, you need to be able to look ahead the two or three moves that include all the captures. But you also need to be able to judge who has gotten the better of the deal after the entire series of exchanges. For that, you need to go back to simple counting.
Black threatens to capture three times on d4, but White has only two defenders. The play goes 1…. Nxd4 2. Nxd4 Bxd4.
Now everybody can see that the queen shouldn't capture on d4.
