Yamaha YHR-891 G Owner's Manual page 14

Options
E b Crook (optional)
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This optional main tuning slide is designed for the YHR-314II and transforms the single F horn into an E b
horn so that you can play E b parts without transposing.
The crook is designed so that the horn fits in the case when it is attached.
When using the E b crook, the 1st through 3rd
valve tuning slides should be pulled out a little.
30
Main Tuning Slide
E b Crook