The area ‘through the green’ is often misunderstood by golfers. Common misconceptions are that it is; when a ball goes over the back of the green; another way of describing the fairway; another way of describing the rough; or another way of describing the fairway plus the rough.
Here is the Definition from the front of the Rules book;
“Through the green” is the whole area of the course except:
a. The teeing ground and putting green of the hole being played; and
b. All hazards on the course.
Here is an example of how understanding the meaning of ‘through the green’ may affect your play. If your ball lies in the rough, just off the closely mown fairway, and you are taking relief from an immovable obstruction, or an abnormal ground condition (e.g. GUR or casual water), the relief procedures outlined in Rules 24-2b(i) and 25-1b(i), Relief – Through the Green, require that you must drop the ball within one club-length of and not nearer the hole than the nearest point of relief, which must not be in a hazard or on a putting green. So, if the permitted one club-length relief takes you from the rough to the fairway you are entitled to drop the ball onto the fairway (Decision 24-2b/8). Similarly, if your ball lies on the fairway and a Local Rule for Preferred Lies is in effect, you may place the ball onto the first cut of rough, providing that spot in the rough is within the distance you are entitled to prefer your lie (e.g. within 6 inches, or the width of the score card.
It is worth noting that Rule 25-2 only permits players to take relief for a ball that is embedded in a closely mown area.