David's Tip of the Day: Transcribing Tip - Figuring out the Key of Harmonica, Part 3
No matter which technique you use from the list of three I presented on 1/6/2015, if you make an error in your key selection, it's usually because you heard that the 5th of the key sounded good and took it as the root note.
Let me help you to understand this a bit...
There are seven notes in a key (C D E F G A B for example in the Key of C)... eight if you include the root note resolving to its octave at the end of the scale (C D E F G A B C). If you play a note not of this scale, it will jump out as not matching at all... this is easy to hear. If you play a note of the scale, it will match, but some match more than others. The ones that match the most are part of the chord built from the first note of the scale, the "one" or home chord (C E G in this case). The Root of the chord is C, the 3rd of the chord is E (third note up in the scale from C), and the 5th of the chord is G (fifth note from C).
The Root of the chord (C), which is also the key of the song, will match the most, but the G (5th of the chord, or 5th note of the scale) will sound really good as well... it won't match as well as the root, but it comes REALLY close... so close that you may stop searching and end up with the wrong key.
The reason for this involves a little science of music...
When a string is plucked (guitar string for example), or column of air excited (trumpet for example) you not only hear the fundamental note (C in this case) but also a spectrum of notes called overtone, partials or harmonics (different names for different contexts). It's similar to the idea that white light contains a spectrum of color, and so does each sound you hear. This brings us to the fact that the first overtone/partial/harmonic you hear other than C is G, the 5th above the fundamental vibrating note... making the fifth of a key sound very alluring... often being mistaken for the key of the song. In this case of the song being in the key of C, you may have thought it to be in the key of G (1 2 3 4 5 = C D E F G).
So... if you think you got the right key and pick up a harp to match and it doesn't sound quite right, go down four notes in that scale and try that note (G F E D C for example) and most likely you'll be good to go.
Here's a good description and picture of overtones on the staff: http://dictionary.onmusic.org/terms/2459-overtone
Have a great weekend!