This is a question  I have seen come up in online forums several times, and it is inevitably followed by a debate about which side of the horn is best to use in a warm-up (or in general). Here’s a quick recap of the major benefits to using each side. There are certainly more good points that could be made, but I think this list hits on most of them.

F Side Benefits

• Warming up on the longer horn, with its greater resistance and closer partials, can translate into greater accuracy, endurance, flexibility, and clarity when performing on the double horn.
• The tone produced on the F horn is considered by some to be the ideal horn sound. Thus, spending time each day playing exclusively on that side of the horn helps to solidify one’s sound concept.
• Warm-up patterns which utilize the harmonic series (Farkas, Dufrasne, etc.) help us become more familiar with the tendencies of each harmonic. Furthermore, striving for correct intonation and a pure tone on less stable valve combinations (1+3, 1+2+3) makes it easier to play those pitches in tune when using conventional fingerings. This is also applicable to a certain extent for the B-flat side.
• If your horn normally stands in F (with the thumb lever up), playing on the F side in the high range can help relieve excess tension in the left hand.

B-flat side Benefits

• Becoming fluent with B-flat fingerings and intonation tendencies opens up several fingering possibilities for both stopped and open notes.
• Striving to create your ideal sound on the B-flat horn can help you learn to match tone quality on both sides of the horn.
• Playing in the high range on the fingerings you would actually use in performance (B-flat side for most double horn players) helps reinforce the kinesthetic memory for those pitches.
•  If your horn normally stands in B-flat (with the thumb lever up), playing on the B-flat side in the high range can help relieve excess tension in the left hand.  Although this has traditionally been associated with European players, I am aware of several American horn players who have made the switch.
• Familiarity with B-flat fingerings through the entire range is a necessity when playing single B-flat or double descant horns (B-flat/high F or B-flat/high E-flat). And while the single F horn is rarely considered a viable choice for a professional quality instrument, single B-flat and descant horns are.

After reading the above list you might be wondering “Well, which is it – which side should I be using in my warm-up?”  I think the reality is that playing the double (or triple) horn means using both (or all three) sides, with the goal of being as fluent as possible going from one side to the other over the entire range. In my opinion the best way to get there is to use both sides of the horn in the warm-up, perhaps even alternating from one day to the next which patterns you play on the F and B-flat sides. In this way you can reap many of the benefits without sacrificing technique on either side of the horn. We often get so locked into our own personal warm-ups that it’s easy to zone out during what is arguably the most critical part of our practice regimen. Building in some variety is a great way to make sure that we are maintaining our awareness as well as our chops. I also periodically play an entirely different warm-up, something totally different from my regular routine. I find that after doing this I return to my regular routine with a fresh perspective. If you play mostly on the F side in your warm-up, try something that emphasizes the B-flat side, like Warming Up, by Ifor James. Conversely, if you use the B-flat (or both sides) regularly, try something that relies heavily on the F side, like the Farkas warm up or the Dufrasne Routine. Another noteworthy collection is Daily Routines for Horn, by Marian Hesse. This collection includes 8 different routines, most of which work the F and B-flat sides equally.