Golf course designers artfully take advantage of the natural terrain to create challenges for golfers of all skill levels.
If you play on a course with rolling fairways you can cope with shots off of sloping lies through simple adjustments to your stance and swing. When the golf ball is below -- lower than -- your feet, it's referred to as a sidehill lie. Maintaining your balance while swinging at a ball that lies on a downslope is difficult -- and balance is critical to making solid contact with the ball.
Aim left of your target (for a right-handed golfer). Depending on how much slope you're dealing with, the ball will tend to move from left to right -- a fade -- in flight.
Take a shorter swing than normal so you can stay balanced and make solid contact with the ball. Slow down your tempo to ensure you don't lose your balance.
Choose a longer club for the distance to compensate for the shorter swing. For example, if you would normally hit a 7-iron from that distance, choose a 6-iron.