Chapman

Both partners tee off. Then they switch balls: A plays the second stroke with B's ball and B with A's. After those second strokes they choose one ball, pick up the other and play it by alternate shot until it is holed (Committee Procedures, 9B). Because they alternate, the third stroke with the chosen ball is played by the partner who teed off with it.

• Both drives out of bounds or lost, before the choice.

  Neither the Rules nor Section 9B settle this case: the terms of the competition do. In keeping with the format, where each ball is played in turns and a penalty does not change the turn (Rule 22.3), both partners tee off again, each with the other's ball, playing 3 under stroke and distance (Rule 18.2b); then they choose one ball and keep alternating. Unlike greensomes, in Chapman the choice only comes after the second strokes.

  Not set by the Rules: the terms of the competition decideR&A, Committee Procedures, Section 9: Other Forms of PlayRules of Golf (R&A), Rule 18: Stroke-and-Distance Relief, Ball Lost or Out of Bounds, Provisional BallRules of Golf (R&A), Rule 22: Foursomes

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• One drive out of bounds or lost and the other in play, before the choice.

  The Rules and Section 9B do not settle this either. In keeping with the format: with the ball that went out, the partner plays 3 from the teeing area under stroke and distance (Rule 18.2b), and the other ball gets its crossed second stroke as usual; then you choose. Some Committees let the side choose the ball in play straight away: check the terms.

  Not set by the Rules: the terms of the competition decideR&A, Committee Procedures, Section 9: Other Forms of PlayRules of Golf (R&A), Rule 18: Stroke-and-Distance Relief, Ball Lost or Out of Bounds, Provisional BallRules of Golf (R&A), Rule 22: Foursomes

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• A drive may be out of bounds or lost.

  In keeping with the format, the provisional for that ball is played by the partner, who would make the next stroke with it, as Rule 22.3 requires in foursomes. Check the terms of the competition.

  Not set by the Rules: the terms of the competition decideR&A, Committee Procedures, Section 9: Other Forms of PlayRules of Golf (R&A), Rule 22: Foursomes

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• After choosing the ball.

  Alternate strokes as in foursomes; a penalty stroke does not change the turn (Committee Procedures 9B; Rule 22.3).

  Under the RulesR&A, Committee Procedures, Section 9: Other Forms of PlayRules of Golf (R&A), Rule 22: Foursomes

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Not a form of play of its own in the Rules: Rule 21.5 lets them be adapted and the R&A describes it in its Committee Procedures (Section 9B) as a variation of greensomes, under the name Pinehurst Foursomes. Whatever that does not cover is set by the terms of the competition.