Heckel (90b) / Fox 25

Low-C#+D# trill on the long-joint

What: A single L-4f key (normal Eb-key) for low-C# and low-D# (there is no low-C#-key)

C#+D# trill: C D ● ● ● Eb / E ● ● ● F

Use:
Facilitates trilling and playing low-C# and low-D# consecutively in fast passages.

Notes:
It is not possible to play low-E with the low-C#-key.
There is no low-C#-key to use as a resonance-key.