The bis key on a saxophone allows you to roll the finger used to sound B so that you can play Bb with one finger. When I went to school, I was taught to use the first finger of each hand to finger Bb. Thirty years later when I started up on the sax again, I was admonished not to use that fingering, but rather to use the A fingers with the side Bb key.
I couldn’t believe anyone would recommend that but then I played a number of vintage Selmer Mark VIs and discovered that my learned fingering (1 + 1) resulted in a very bad intonation for that note! Could it be that is what drove instructors to recommend the A + side palm keys?
So I showed one of my instructors that on my sax, the Couf Superba I, I could play both fingerings and the intonation was relatively the same. Casa Valdez Studios sez it in even a more compelling manner the post Bis Key and Saxophone Hell. And the picture above is of a Oleg add-on to make it even easier to use the bis key.
But if I started purchasing one for each of my saxes, like I did the Vandoren Optimum lig, (ooh, one screw adjustment) I’d need five for my main saxophones. And then my wife would want them for her two saxes. ;o)
About Gandalfe
Just an itinerant saxophonist trying to find life between the changes. I have retired from the Corps of Engineers and Microsoft. I am an admin on the Woodwind Forum, run the Pacifica Big Band (formerly the Microsoft Jumpin' Jive Orchestra) and participate in other ensembles. Mostly enjoy time with family and friends.
The Bis Key Chronicles 
The music is quite enjoyable and relaxing. It is wonderful that you and your wife are both musicians. My Ex used to say that I was atonal, which is true, and it was a miracle that I was in our church choir…lol. I may not be able to sing on key, but I can appreciate good music, and do enjoy jazz.