If I play C E G on the piano, then G acts as a perfect 5th, but if I play G C E and the lowest note (just below the C) is G, can I still call G a perfect 5th of C maj? Because the interval now between G and C is 5 semitones i.e. perfect 4th.
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2 Answers
Watch out for the distinction between an interval above a pitch and the chordal member. G is a perfect fifth (an interval) above C, but it's also the chordal 5th of a C chord.
Intervals are always measured from the bottom note. So when you measure the interval from C up to G, you always count in that direction.
But when you place the G below the C, now you count the interval from G up to C, thus (G–A–B–C) it's a fourth.
You've stumbled upon what we call intervallic inversion and its corollary the Rule of 9. In short, when you invert an interval, you move the upper pitch so that it's now on bottom; the resulting interval is the original interval subtracted from 9.
In other words, you started with a fifth from C to G. When you invert that interval, it becomes a (9 - 5 = ) 4th.
There's also a pattern of qualities and how they change during intervallic inversion:
Major becomes Minor, and vice versa
Diminished becomes Augmented, and vice versa
Perfect stays Perfect
But if you're not identifying interval qualities yet, don't worry about it!
answered Aug 22, 2018 at 15:29
Richard is stepping down♦Richard is stepping down
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"Intervals are always measured from the bottom note. So when you measure the interval from C up to G, you always count in that direction." I suppose that this is technically true, but it's a strange way to phrase it. Saying F is a perfect fourth above C is the same as saying C is a perfect fourth below F. It doesn't matter where you're starting from: a perfect fourth is five half steps. Five half steps (a perfect fourth) above C is F; five half steps (a perfect fourth) below C is a G.– John DoeAug 22, 2018 at 20:35
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@Richard Please tell us more about the patterns (rules) for identifying Major to Minor, Dim to Aug, etc. Aug 22, 2018 at 21:31
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@StephanLuis You may interested in Dom's answer at music.stackexchange.com/questions/41278/… Aug 23, 2018 at 20:29
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@JohnDoe While you're correct, my experience is that it's safer pedagogically to always understand intervals as being counted from the lower pitch. Furthermore, it's more pedagogically sound to stay away from half-step counting with tonal intervals; that approach often leads to error and confusion regarding enharmonic intervals. Aug 23, 2018 at 20:31
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Yes; I wasn't suggesting using half-steps as the primary means of determining interval size. I was merely using it to point out that an interval is the same length, no matter if you're going up or down. RE: the other point: What in the world does "pedagogically safer" mean? Your description sounds like it's the Common Core of music instruction: instead of knowing that you can move up and down by specific intervals, you're forcing a change of reference. I do believe it's important to be able to count intervals from the lower pitch, but "always"? A music student should be able to go both ways.– John DoeAug 23, 2018 at 20:42
What is confusing you is that a note by itself isn't the start point for intervals. We always count up from the lower note. Thus, the distance between C and G isn't the same as that between G and C. Sounds confusing? Once you've established which note is lower, then count up to give the number relevant to the interval. C-D-E-F-G gives a perfect fifth (P5), whereas G-A-B-C gives a perfect fourth (P4).
An interval is the space between two given notes. So, yes, using the lower one as C, C>G is a P5. With G as the lower note, G>C is a P4. Note (sic) the 'rule of 9'.
Any two notes will have two different interval names, depending Which you consider the lower. Thus, C>E is M£, while E>C is m6.
And - don't be fooled by merely counting semitones. C>E is M3, while C>Fb is diminished 4...
