The continuous curve starting at the break point and ending at the tip rail. The facing curve can be seen on a mouthpiece by looking at it from the side. This curve varies greatly among mouthpiece brands and plays a crucial role in the response the mouthpiece gets. It is very important that the facing curve on both of the mouthpieces 'side rails' be parallel or even (unless made intentionally uneven for a specific purpose. I know some Refacers experimenting with this). It is also important that the curve be continuous and gradual, with no flat spots or bumps.