This is a description of how WinTD implements the ratings-based Swiss System using US Chess methods. First, it's important to understand that there is no way to guarantee that any program will find the "best" pairings in every situation. If you look at a 20 player score group, there are 3,628,800 (10!) ways to pair the ten top half players with the ten bottom half players. Examining every single one of those to find the "best" would take much longer than your players would like to wait.

 

The US Chess rules allow for the possibility that there will be more than one valid set of pairings. Suppose, for instance, that a score group can be paired with all colors correct in two different ways: in one, it requires both a 40 point swap and a 30 point swap, the other requires a single 45 point swap. Which is better? Either is acceptable, and no human TD upon finding one reasonable solution would likely keep looking to see if something a bit better was available.

 

Most pairing programs and most human TD's pair by starting at the highest score group and working down. Where there is an odd number of players in a score group, or some other situation which prevents all the players from being paired within the group, the odd player or players are dropped or "downfloated." On occasion, the choice of whom to drop causes serious problems in a lower score group. A very good human TD will recognize the problem and go back up and change the dropped player to smooth the pairings out. However, the typical behavior of a pairing program is to push forward, even if it requires multiple drops at the lower level.

 

WinTD uses an approach which looks at the section as a whole. We have constructed a "penalty" function which measures how far the pairings differ from ideal pairings. What are "ideal" pairings?

 

•Everyone is playing someone with the same score.

•No previous pairings are duplicated and no pairing preferences (such as teammate or clubmate avoidance) are violated.

•In every score group, the top half players are playing the bottom half players in order.

•On every board, the colors are "right," that is, everyone has as close to an equal number of whites and blacks as possible, and the alternate color from the preceding round.

 

The penalty value for ideal pairings is 0, which is almost impossible to achieve after the first round. The penalty function is designed so that lower values are "better" in the sense of being closer to ideal. For instance, the highest penalty is assigned to a pairing that has two players playing each other a second time. A lower penalty is assigned to a pairing which crosses score groups. A still lower penalty is assigned to pairings which have incorrect colors. And a rather small penalty is applied to pairings which change the natural pairing order slightly and involve a small rating change.

 

By finding a set of pairings which gives the smallest value for the penalty, we should end up with pairings which come as close to ideal as we can get. However, as noted above, there is no way that any program can look at all possible ways to pair any section with more than a very small number of players. Instead, WinTD uses the power of modern computers to find a "good" set of pairings. In testing this against other pairing programs, we found that WinTD almost invariably found superior pairings: more correct colors with smaller rating changes. In simple situations where a straightforward pairing process would work fine, WinTD would find the same solution.

 

There are three levels at which the TD can control the pairing process. The Pairing Rules in the Preferences Dialog Box deals with questions which are mainly ones of personal taste. In the Pairing Rules tab of the Add/Edit a Section dialog box, there are choices which are likely to vary from tournament to tournament or section to section. And finally, in the Pair a Round dialog box, there are several choices which might very well change from round to round.

 

Ranking Players within a Score Group

 

Within a score group, the players are ranked in terms of their pairing numbers. However, when players have exactly the same rating (this applies mostly to the unrateds), they are reshuffled among themselves each round. That way, no unrated will, simply by virtue of an arbitrary initial ordering, systematically get either easier or harder pairings than other unrateds.

 

See Pairing Numbers, Color Assignments, Color Corrections, Assigning Byes, Accelerated Pairings, Drop/Raise (Float) Decisions, Small Tournaments and Teams and Clubs for more information on those aspects of the pairing process.

 

Copyright © 2026 Thomas Doan