Wrong Green
A wrong green is a green-like area next to the actual green that does not belong to the hole being played.
Typical wrong greens are practice greens at the edge of the course, the green of another hole in the case of double greens, and abandoned or newly created green surfaces. They are mown and maintained just as short as the real green, only they do not belong to your hole under the rules. The rulebook treats the wrong green more strictly than other areas: you may not play from there, even if the lie is tempting. A putt across the wrong green, on the other hand, is allowed as long as your ball is not lying on it.
In practice the matter is simple. An example: your approach shot slides over the green and stays on the practice green behind the hole. You mark the spot, pick the ball up and look for the nearest point in the general area that is not closer to the hole. From there you drop within one club-length, without penalty. This relief is obligatory, not an option. Whoever plays from the wrong green anyway takes two penalty strokes in stroke play.
The typical mistake is the assumption that the ball may stay where it is because the surface looks like a green. That is exactly the point: the rulebook wants to prevent putting surfaces from being damaged by shots with irons and wedges. Remember the simple rule: if the ball lies on a green-like surface that does not carry your flag, drop beside it without penalty. With double greens the following applies: only the part with your flag is your green.
FAQ
May I play my ball from the wrong green if the lie is good?
No, the rulebook expressly forbids it, even if the lie is tempting. If your ball lies on a wrong green, you have to drop without penalty: nearest point in the general area, not closer to the hole, within one club-length. Whoever plays anyway takes two penalty strokes in stroke play — this relief is thus obligatory, not an option.
What should I watch out for with a double green?
With a double green the following applies: only the part with your flag is your green, the other part counts as a wrong green. If your ball lies on the wrong half, you take free relief as with any other wrong green. If your ball only putts across the other half without lying on it, that is allowed.
Why is playing from the wrong green forbidden at all?
The reason is course care: practice greens and secondary greens should not be damaged by shots with irons and wedges, which would leave deep scars there. The surfaces look like a real green but do not belong to your hole under the rules. Rule of thumb: if the surface does not carry your flag, drop beside it.