Auxiliary F key (aka aux F or front F)
The top F auxiliary key (ie the key just above the B key) is an alternative way to finger top F. As you can see in the fingering chart, this involves fingering a top C (2nd LH finger) and adding the front F key. This allows a very quick way to leap from C to F.
But this extra key was only added to the saxophone some time in the 1920s. Before that if you wanted to transition quickly from top C to top F (or any of the left hand fingerings), it was quite awkward because you’d have to open the palm keys. The problem is it’s not that easy to open all the palm keys simultaneously. It often tends to gliss through the D and E. This is fine if you want a sort of glissando or grace note effect but not too good for a clean C to F.
Here’s the good news: you can fake it.
This diagram shows what actually happens , ie which keycups are closed when you use auxiliary F key along with a C fingering (as shown in the first diagram).
To test this, finger a top C. Keep your 2nd LH finger on the C and close the aux key, you will see that the aux key closes the LH1 key (B) and opens the palm F key. So this is basically an A fingering but with the F palm key open.
In this case you might think that the F palm key is making the F sound. This is true, but not in the same way it does using the conventional palm fingering because that relies on the keys below (E, Eb and D) also being open.
In this case the F palm key is merely acting as a vent (rather like the octave key) and is causing an overtone of the A to sound.
So in order to replicate this without the auxiliary F?
All you need do is finger A, and use your third LH finger (ring finger) to open the palm F.
It is slightly more awkward but you will get the hang of it quite quickly and it can come in very handy at times.
Note that it is quite an easy modification for any competent technician to add an aux key, but many people like to keep their vintage instruments original.
Not that although you would use the octave key for top A or C, it is not necessary fro the aux F or our “fake” aux F.
This is because the octave vent “interferes” with the F vent and causes it to not work quite so well as when you don’t press the octave key. In real life situations it is not so easy to remember this, and so we do tend to keep the octave key pressed for top (aux) F especially in faster passages. But if you can remember to release it you may find it useful for sustained top F either with aux F or our “fake” fingering here.
NB: as you will probably realise when you try this, you do need you first finger to also cover the bis Bb. It’s only like that in the photo because I was reaching around with my other hand to take the “selfie” with nay iPhone.
However, if you do have your finger on the bis Bb you will be able to move easily to F# by taking your second finger off the A key. Without the finger on the bis you similarly can move easily to G.
See top F to G trill