The Complete Idiot''s Guide to Music Theory

Part 4:
Accompanying
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Maybe you’re playing organ in the church choir. The guest vocalist this week
approaches you just before Sunday’s service, and says she’ll be singing “Amazing
Grace,” in the key of G. She doesn’t have any sheet music for you—“You know
the song,” she says—and if you don’t, you’ll have to learn it, fast.
Perhaps you’re playing saxophone in a pickup band at your local bar. It’s open
mike night, and the next performer wants you to add some horn accompani-
ment to his tune. “It’s a 12-bar blues,” he says, “in the key of A. You have a solo
after the second chorus.” No music, no nothing—just you, onstage, winging it.
All of these situations require you to think on your feet—or, more properly, to
play by ear. You need to be able to hear a song and figure out what chords and
notes are being played, without benefit of any written music.
This might sound daunting to you, but it’s a skill you need to develop. You have
to be able to hear a melody or chord progression, and then 
transcribe
what you
hear. With practice, you’ll be able to do this quickly and accurately; it can even
become second nature. All you have to do is train your ears.
Transcribing music involves three distinct steps:
1. Listen
2. Process
3. Document
You begin by listening to the music—not for passive enjoyment, but more actively,
so that you clearly hear every note. Then you process what you’ve heard, figur-
ing out what notes and chords and rhythms are being played. Finally, you docu-
ment the music you’ve processed, either by writing down the notes on paper, or
using the processed music as a basis for live playing or improvisation.
With practice, you’ll move through the listen-process-document system almost
subconsciously; the notes will come to you automatically when you hear them,
without a lot of work on your part. So warm up your ears—it’s time for the
training to begin!
Transcribing
is the
art of writing down melo-
dies and chords by ear,
without the benefit of any
other written notation.