The facing is the continuous curve starting from the break and ending at the tip rail and is the surface the reed vibrates against while the musician blows into the mouthpiece.
 
The facing varies greatly among mouthpiece brands and dictates how free blowing or resistant a mouthpiece is, as well as the type of response the mouthpiece gives in the upper, middle, and lower registers of the instrument.
 
Selmer and Brilhart use short facings and make the upper register easier, Otto Link and Meyer use medium facings as do most other manufacturers and the New York Otto Link and Berg Larson use a long facing which accentuates the lower end of the horn.