Multiphonics
Multiphonics is the act of producing more than one pitch simultaneously on the horn. To do this, one note is produced as normal while another is sung. Doing this it is quite difficult to produce an aesthetically pleasing sound, but nonetheless can be done. Like other wind instrument techniques, it is not unique to the horn. One of its earliest uses, however, occurs in the Concertino for Horn and Orchestra by Carl Maria von Weber.
Another kind of multiphonics can be achieved by simultaneously playing two neighbouring notes of the harmonic series. A practical way of doing this is by placing the lower lip under and outside the mouthpiece, playing one note, and then gently, by increasing air pressure and adjusting one's lip-position, halfway slurring upwards to the next harmonic step. This might be frustrating at first, and the technique is quite an unstable one to perform in real-time, especially when compared with similar practices with other brass instruments, esp. trombone. This technique finds its occasional use in contemporary music where, successfully performed, it might evoke an interesting effect.
Read more about this topic: List Of Horn Techniques