In Mac-Mahon system, pairing and placement are made according to Mac-Mahon score.

Mac-Mahon score

At the beginning of the tournament, each player is given a starting Mac-Mahon score (SMMS) which is based upon his ranking.
For instance, a 1 dan player starts with 30 points, a 20 kyu player starts with 10 points.
After each round the MMS of each player is increased by 1 for a win, 1/2 for a draw, 0 for a loss.
If a player does not participate in a round, he gets 1/2 point.
Final Mac-Mahon score determines the placement.

Mac-Mahon bar

Starting Mac-Mahon scores are limited by a floor (typically 15 kyu or 20 kyu) and a ceiling, also called Mac-Mahon bar (typically 4 dan for big tournaments).
The bar should be chosen low enough to accept as many players as possible in the top group and high enough to enable a clear final placement.
In some big tournaments, above the top-group, a super-group and a super-super-group may be added.

Pairing

Pairing is done inside a MacMahon group, that is to say between players with the same Mac-Mahon score.
Usually, players with a non-integer MacMahon score are paired with the group just below.