The C# trill key is an extremely useful mechanism which is gaining popularity among flutists. As shown in the diagram below, the C# trill key is situated along the trill rod. By depressing the spatula near the Bb bis key (shown by the arrow), the C# trill key is opened along side the thumb keys.

The C# trill key brings increased versatility to many trills and tremolos, including B-C# and the third octave G-A trills. Additionally, the C# trill key can be used to provide a more stable and colorful middle C# when activated while B-natural is fingered. The following is a guide to illustrate some of the many functions of the C# trill key:
| Trills |
| > |
B-C (first and second octaves): Finger B and trill the C# key. |
| > |
C-C# (first and second octaves): Finger C and trill the C# key. |
| > |
High F#-G#: Finger high F# and trill the C# key. |
| > |
High G-Ab: Finger high G and trill the C# key. |
| > |
High G-A: Finger high G and trill the C# trill key and D trill key in unison. |
| > |
High Ab-Bb: Finger high Ab and trill the C# trill key, the D trill key and the D# trill key in unison. |
| Tremolos |
| > |
In the first octave, tremolo to C# from G, Ab, A, Bb, B or C by trilling the C# trill key. |
| > |
In the second octave, tremolo to C# from A, Bb, B or C by trilling the C# trill key. |
| > |
In the first octave, tremolo to D from G, Ab, A, Bb or B by trilling the C# trill key and the D trill key in unison. |
| > |
In the first octave, tremolo to D# from G, Ab, A, Bb or B by trilling the C# trill key and the D# trill key in unison. |
The C# trill key is available on all models with the exception of the PA-102 and must be installed on the flute during manufacture.