[CompSci87]

If one is not sure that his or her ball has is lost in a water hazard or outside it (i.e. in deep grass nearf the hazard) he or she is not allowed to play a provisional ball from the tee.  This is one of the least known and most arcane elements of the Rules of Golf.  I learned this (to my detriment) whilst playing in a USGA competition.

Not a golfer, but I have to ask: what if you're sure it's lost, but then someone finds it after all? And what's a "provisional" ball anyway?

[CompSci87]

If one is not sure that his or her ball has is lost in a water hazard or outside it (i.e. in deep grass nearf the hazard) he or she is not allowed to play a provisional ball from the tee.  This is one of the least known and most arcane elements of the Rules of Golf.  I learned this (to my detriment) whilst playing in a USGA competition.

Not a golfer, but I have to ask: what if you're sure it's lost, but then someone finds it after all? And what's a "provisional" ball anyway?