Pairs (2)
This page looks at pairs as candidates for future sets.
The Nature of Pairs
Pairs have one important characteristic:
their value as set candidates is highest when you have exactly two pairs.
Let us look at some examples.
Example 1 
In a hand like Example 1, where you have only one pair,
has only two tiles of acceptance, so it is not very promising.
And even if you do draw one more, you then lose your head.
(
is an effective tile, but it does not change the shanten number.)
Example 2
But once you have two pairs, as in Example 2, those pairs really begin to function as set candidates.
Compared with Example 1,
the acceptance into iishanten has increased by four tiles: .
When you have two or more pairs, each extra pair adds two tiles that can turn it into a triplet. That kind of acceptance is called a shanpon acceptance, or shabo acceptance.
Example 3 Tsumo
So what happens once you reach three pairs, as in Example 3?
Adding one more pair only adds two more tiles of shabo acceptance. But from Example 3 onward, if you start breaking a closed wait in order to keep increasing the number of pairs, you immediately lose four tiles of acceptance. So beyond this point, increasing the number of pairs becomes disadvantageous.
From a tile-efficiency perspective, the answer in Example 3 is to cut .
As for which pair should be broken first, that belongs to Chapter 2.
(For now, it is enough to understand that three pairs are already starting to become a disadvantage.)
Because each shabo acceptance adds only two tiles, once you reach four pairs or more, the hand's overall acceptance becomes narrow. That is why hands with many pairs often shift toward calling pon or aiming for seven pairs instead.
Theory and Summary
Original Japanese page: http://beginners.biz/kihon/kihon07.html