Having made groups (according to the Mac-Mahon score in Mac-Mahon system or to the number of wins in swiss system), then, in each group,

A seeding is made

to order players. The seeding criteria is based upon secondary criteria (for instance SOS then SOSOS).
For the first two rounds where SOS is not significant, seeding by rating can be considered.
The group is usually split into two parts. Then each player of the upper part is paired with a player of the lower part. This can be done by split and random, split and fold ( 1 - n, 2 - n-1, etc..) or split and slip ( 1 - n/2+1, 2 - n/2 +2, etc..) .

Pairing criteria

The basic split and... system has to be adjusted to take in account additional constraints like :

• avoid two players to play twice together in the same tournament.
• balance White and Black
• avoid games between players with a big level difference.
• avoid handicap games.
• avoid games between two players from the same club (or the same country).

The relative weight of the secondary criteria is very varied according to the tournament.

Odd groups

When a group is odd, one player has to be paired within a higher group or a lower group.
A frequent choice is to pair one of the first players in the upper group with one of the last players of the lower group. But if the problem occurs several times in a tournament, a good balance should be searched for a given player.

But, how does it work, precisely ?

You can find additional information in : OpenGotha User's guide. Look at "Pairing algorithm" appendix.