This is one of those passages that requires precise reading:
Quote:
(Merry) "I am going to blow the horn of Rohan..."
...a sudden clear horn-call...
Then he heard Merry change the note, and up went the Horn-cry of Buckland...
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Merry knew the notes that made up the special signal for the Horn-call of Buckland, and that would have been familiar to many Hobbits. He did not change the
horn, he changed the
notes that he played. A typical hunting horn cannot play all notes of the scale like a modern concert horn can, but it still has enough notes, which can be varied in length and in the order in which they're played, to make different calls.
Imagine a modern military bugle - the same instrument can play reveille, retreat, or taps.
Interestingly, the horn is the one instrument played by humans in Tolkien's works which has more than an entertaining or signalling function - it seems to move the hearts of people in an almost supernatural way. We never read of trumpet signals having a similar effect.