Fragment for Piano in A with Effect of Horns, Biamonti 268
Beneath the bass part, Beethoven has written, "Hier muss der Effect von Hoernern hervorgebracht werden (Here the effect of horns must be emphasized).
The two lowest lines have: E/B, A/C sharp, E/B, etc. These E/B's are what we call "horn fifths". Concealed fifths like these were strictly speaking forbidden, but given the limited number of notes natural horns could play in the 18th century, they were accepted between two horns.
Thus the horn fifths became characteristic for the horn-sound, and even
in music without horns, horn fifths were used to imitate horns! There are many
examples of that LvB's piano music, and Biamonti 268 is yet another one. The Allemande for Piano WoO 81, as well as Hess 60 and 61 and Biamonti Nrs. 268-284 are all found on two sketch sheets now kept in the Beethoven-Archiv in Bonn. The quantity of diverse material that can be crowded into such a small space is truly remarkable!
Biamonti: 268