I've long wondered how song-writers manage it — avoiding tunes that have already been written, that is. The easy answer, of course, is that they don't, at least not always, as we know most famously from George Harrison's trouble with "My Sweet Lord" (who turned out both to be and not to be "So Fine"). Putting aside deliberate sampling, which is a recent and overt pushing of legal and conventional limits, the real worry, I suppose, is unconscious plagiarism. What protections against unwitting trespass does the poor composer have?
Three practical "bulwarks" come to mind. The first is pure mathematics and suggests that your chances of duplicating are poor. Assuming a 12-tone scale, such as the one used in the West, and assuming a melody of just 12 notes, the number of possible permutations and combinations is, well, astronomical. 1 x 1212, in fact, which comes to 8,916,100,448,256, or 8 trillion, 916 billion, 100 million, 448 thousand and 256 (thank you WolframAlpha). I've not accounted for the length of the notes or indeed any other feature that goes to make up "music."
Others far better than I have done similar calculations taking further variables into account. If numbers interest you, take a look at "A Mathematical Look at Musical Plagiarism" and at a forum discussion titled "How many melodies are there in the universe?".
The second protection for our anxious composer is, paradoxically, lack of originality. If your theme was first written by Beethoven or Bach, you're free and clear. If you can torque it enough, you might even be able to copyright your twist, as the composers of "A Lover's Concerto" did by, among other things, altering Bach's "Minuet in G major" from 3/4 to 4/4 time. The borrowing continues to the music of today.
Finally, computer technology might come to help. If you've got, or want, a smart phone, you likely know about Shazam, an app that identifies melodies you play, whistle, or hum for it. In that kind of technology, it seems to me, lies true salvation for the worried tunesmith: a database of a very large number of published melodies, accessible over the internet, would do wonders to reassure composers. It may be that there is such a thing and I just don't know about it; the U.S. Library of Congress would be a logical place for such a service, but nothing like it exists there that I can find.
You might like to play with a couple of these "melody finders" accessible on the web. Musipedia's Melody Hound lets you input your tune via keyboard, microphone, or a mouse. You can even give the program simple "up" and "down" instructions. MelodyCatcher is simpler in structure. Both seem to offer up "classical" music more readily than popular music — perhaps because of copyright concerns.
