O.B. The least favourite letters in the golfer's alphabet.
There is no coming back from here even if you find your ball - a ball played into it may not be played out.
The land is often private and golfers are discouraged from going to look for their ball.
Out of bounds is usually marked by white stakes.
It can also be marked by a wall, fence or stream.
The ball must lie on the inside of a line running along the inside of the posts.
When it is defined by a line on the ground, the line itself is out of bounds.