Most modern drivers, fairway woods and hybrid golf clubs feature a “bulge” designed into the face.
As illustrated below in red, bulge is a convex curvature spanning from the heel to the toe.
Clubs are built with bulge to induce “gear effect” on off-center contact. When the ball misses the club’s center, the head rotates and imparts sidespin on the ball. For example, contact on the toe area causes hooking sidespin, sending the ball left of target. The face’s bulge causes the ball to start to the right, compensating for the spin and delivering a straighter shot.
This effect is illustrated below, in red.
Bulge can help with shot correction, but there is also downside. For the golfer, looking at a curved face makes it much more difficult to accurately aim the golf club at the target. If you tend to focus on the heel area when setting up, the clubface will actually end up open. Similarly, if you tend to focus on the toe area, the face will be closed.
This problem can be solved by golf clubs with a clear and accurate shot aiming indicator designed into the top of the clubhead. (For example, the gold indicator shown in the picture above is positioned on a flattened crown to ensure it is straight and parallel to the ground.) Focusing on the alignment aid, instead of the clubface, prevents misalignment due to bulge.
All clubs made by Thomas Golf feature a patented, built-in alignment guide.