Schematic images of the interrelationship between the mouthpiece bore axis, BA, Feature Line, FL and Tongue Line, TL, The Feature Angle is denoted, FA and Working Angle, WA. The angle that the feature line subtends to the tongue line is denoted as FL. The important angle to determine the relative orientation of a feature line to the tongue line is the TA and this can determined from: TA = WA - FA