A round robin is a tournament format where each team plays against each other team once. In a round robin, 2n - 1 rounds are needed for 2n teams or 2n - 1 teams.
Round robins are the building blocks for many invitational tournament formats. An experienced tournament director will be familiar with the round robin structure, and can set up a round robin on the fly to handle a last-minute change in the number of teams.
An advantage of round robin play is that teams in the same round robin pool play against the same opponents, so there is less concern with strength of schedule.
How to set up a round robin
In its simplest form, a round robin can be constructed for any even number of teams as follows. In this example, we will show a 10-team round robin.
First, place the teams as follows:
Each column represents a first round pairing. To determine the second round's pairings, leave team 1 fixed and rotate the other teams clockwise. (That is, move the other teams on the top row one column to the right, move the top right team to the bottom right, and move the teams on the bottom row one column to the left. The team that played 1 goes in the top row, next to 1.) So we end up with the following pairing for round 2:
This continues until the final round, where team 1 plays against team 2.
For an odd number of teams, use the same procedure for one more than the number of teams, and choose one team (typically team 1) to treat as a bye. (That is, whoever would play the chosen team gets a bye.)
The simplest way to assign matches to rooms is to have the first column represent one room, the second column a second room, and so on. However, for a round robin with an even number of teams, this can be undesirable because team 1 is kept in the same room all day. There are pre-made round-robin schedules available to spread the teams evenly across rooms as much as possible (which is impossible for four teams).