McMahon System: Chess tournament seeding method
McMahon System
Definition
The McMahon System is a tournament pairing and scoring method in which players begin the event with different “starting points” based on a pre-tournament strength measure (usually rating). Pairings then proceed Swiss-style within score groups defined by these McMahon points. Final standings are determined by each player’s over-the-board score plus their starting McMahon points.
In practical terms, it is like a Swiss tournament in which stronger players begin closer to the top of the scoreboard, reducing early mismatches and getting top contenders to face each other sooner.
How it works
- Set bars (rating thresholds) and assign starting points. Example: 2000+ starts with 2.0 points; 1800–1999 starts with 1.0; below 1800 starts with 0.0.
- Round 1 pairings are made within each McMahon score group (e.g., 2.0 plays 2.0, 1.0 plays 1.0, etc.). If a group has an odd number, a player “floats” to the nearest adjacent group.
- After each round, a player’s McMahon total = starting points + their over-the-board (OTB) points so far. Pairings continue by current McMahon totals, just like a Swiss pairs by current score.
- Final standings use the McMahon totals. Usual tiebreaks (e.g., Buchholz, Median-Buchholz, Sonneborn–Berger) can be applied; only game results and opponents influence tiebreaks, not the starting points themselves.
Usage in chess
While the Swiss System is standard in chess opens, the McMahon System is occasionally used by clubs and regional organizers when the field spans a wide range of ratings. It lets everyone play in a single section yet avoids extreme early-round mismatches, and it gets top players meeting each other sooner.
- Common in club weeklies and local weekend events with mixed-strength fields.
- Class prizes can be awarded within the one-section tournament; the McMahon start points help keep the overall competition coherent.
- For rating: rating calculations only use actual game results; the starting points do not affect ratings. For norms and high-level events, organizers typically prefer approved Swiss or round-robin formats—always check the event regulations.
Strategic and organizational significance
- For players: If you start near or above the bar, you’ll face tough opposition earlier; if you start below, expect more even early rounds and realistic class-prize chances.
- For organizers: Fewer lopsided boards in Round 1, better competitive balance, and a single-section event that still respects rating strata.
- Compared to Swiss: Swiss starts everyone at 0.0; McMahon starts stronger players higher. Compared to Accelerated, McMahon uses persistent starting points rather than temporary acceleration in early rounds.
Example setup and pairings
Suppose a 5-round open with these bars:
- 2000+ → start with 2.0
- 1800–1999 → start with 1.0
- Under 1800 → start with 0.0
Ten players enter:
- A (2150) → 2.0; B (2100) → 2.0; C (2050) → 2.0
- D (1950) → 1.0; E (1900) → 1.0; F (1850) → 1.0
- G (1750) → 0.0; H (1650) → 0.0; I (1550) → 0.0; J (1400) → 0.0
Round 1 pairings (within McMahon groups):
- 2.0 group (A, B, C): A–B; C floats down to the 1.0 group.
- 1.0 group (D, E, F plus C): C–D, E–F.
- 0.0 group (G, H, I, J): G–H, I–J.
Suppose A, C, E, G, and I win (and B, D, F, H, J lose). New McMahon totals:
- A: 2.0 + 1 = 3.0; C: 2.0 + 1 = 3.0; B: 2.0 + 0 = 2.0
- E: 1.0 + 1 = 2.0; D: 1.0 + 0 = 1.0; F: 1.0 + 0 = 1.0
- G: 0.0 + 1 = 1.0; I: 0.0 + 1 = 1.0; H: 0.0 + 0 = 0.0; J: 0.0 + 0 = 0.0
Round 2 pairings will group 3.0 vs 3.0 (A–C), 2.0 vs 2.0 (B–E), 1.0 vs 1.0 (D–F and G–I), and 0.0 vs 0.0 (H–J). Notice how the top players face each other immediately in Round 2, something that a standard Swiss might only guarantee later.
Final standings example: If A scores 3.0/5 OTB, their McMahon total is 2.0 + 3.0 = 5.0; if G scores 3.5/5 OTB, their McMahon total is 0.0 + 3.5 = 3.5. A still finishes ahead overall, but G might win a class prize.
Key terms
- Bar: The rating threshold where a starting-point level changes (e.g., the “top bar” at 2000).
- Starting points: The McMahon points assigned before Round 1. They never change except by adding your OTB results.
- Float: Pairing a player up or down into the nearest adjacent McMahon score group to resolve odd numbers.
Pros and cons
- Pros: Fewer mismatches, faster convergence of top boards, single-section flexibility with class prizes.
- Cons: Lower-rated players start “behind,” which can feel discouraging in overall standings; careful bar selection is needed to keep pairings smooth; norms/title considerations usually favor standard Swiss or round-robin.
Historical notes and interesting facts
The system is named after Lee McMahon, who popularized it in the 1960s for go tournaments; it spread widely in go because the field often spans many ranks. Chess organizers later adapted it for local events with broad rating ranges. In spirit, McMahon is a cousin of the accelerated Swiss, but its use of persistent starting points gives organizers a more direct way to “seed” a single-section tournament.
Fun fact: Many players describe McMahon as “paying the Swiss tax up front.” Instead of spending early rounds beating much lower-rated opponents, top players start nearer each other and clash sooner.
Practical tips
- Players: Know your starting points; keep track of your McMahon total to anticipate pairings and tiebreak stakes.
- Organizers: Choose bars that reflect the field’s distribution (e.g., align with rating classes). Publish the bars and starting points in advance and clarify prize and tiebreak policies.
- Arbiters: Handle half-point byes and late joins the same as in Swiss; byes add to OTB points, not to starting points. Standard color and float rules still apply.