Definition of the Putting Green
It is the area with the shortest (very closely mowed) grass on a golf course. The hole with a flagstick is placed there. The cup should be located at least four paces (yards) from any edge of the green.
Collar, Apron and Fringe
The collar and apron grass is slightly higher than the green surface height. It might be the same height as the fairway grass or it might be around the mid-point between the height of the green and the height of the fairway grasses.
A collar nor apron is not a part of the putting green. A golfer has the opportunity to try to run the golf ball from collar or apron onto the green, rather than being required to fly the ball to the putting green’s surface.
Fringe (colloquially frog hair) can be used as a synonym for either collar or apron. It refers to the height and/or type of any grass adjoining the putting surface while collar and apron refer to the areas in front of or around a putting green.
When a ball is on the Putting Green?
A ball is always lying in only one of five areas of the course.
Ball is on the putting green if “any part of the ball touches the putting green or it lies on or in anything and is inside the edge of the green”. This definition applies to the case where we are considering the boundary between the putting green and general area.
When a ball lies on the boundary between the putting green and the bunker the ball is in the bunker if “any part of it touches sand on the ground inside the edge of the bunker“. If the ball doesn’t touch sand it is on the putting green. See the picture below.
update: March 2021