What counts as a stroke in golf?

• Not a stroke: a swing the player deliberately stops before the ball, or deliberately steers past it. Also hitting the ball by accident in a practice swing, though moving a ball in play that way is usually one penalty stroke and the ball is put back.
• Fair strike: only momentary contact between the clubhead and the ball. Any part of the clubhead may be used.
• Double hit: if the club accidentally hits the ball more than once in a stroke, it is one stroke and there is no penalty.
• Anchoring: the player must not hold the club or a gripping hand against the body. Nor may a forearm be held against the body to make a fixed point to swing round. Holding the grip against a hand or forearm is allowed, and so is clothing or the body being brushed during the stroke.
• Standing on the line: the player must not deliberately stand with a foot on each side of the line of play, or on it, behind the ball.
• A moving ball must not be played, with three exceptions: a ball that starts to move after the backswing has begun, a ball falling off a tee and a ball moving in water.
• Help: nobody may physically help the player during the stroke, shelter them from wind or rain or be placed to block a distraction. Players may hold their own umbrella.
• Alignment: nothing may be set down on the ground to help with aiming or the stance.
• Breaking any of these is the general penalty. In stroke play the stroke still counts.

The Rules call the freely swung club the "fundamental challenge" of the stroke. Each ban removes a way of steadying or steering the club that takes skill out of it.