[MUSIC] In the last lecture you learned about generic intervals, that is the distance between two notes described with a number that represents the number of lines and spaces or note letter names separating them. So c to d is a second, c to e is a third, remembering that a note to itself is a first or a unison. But this generic label does not describe the sound of he interval. Since c anything to d anything, sharp flat, double flat, is a second, regardless of what it sounds like. All of the intervals shown here are technically seconds even though they sound very different. In other words the generic interval is a second for all of them, but the actual sound is different for all of them. So here's c to d. [MUSIC] Here's c to d sharp. [MUSIC] Here's c flat to d sharp. [MUSIC] Here's c double flat to d double sharp. [MUSIC] Technically, they're all seconds, even though they sound very different. The interval label based on interval sound is called the interval quality. The interval quality depends on the number of half steps. A second that has one half step is a minor second. A second with two half steps is major second. A second with three half steps is an augmented second, and so on. Let's look at a sample interval. Say C to G. It sounds like this [MUSIC]. The generic interval is a 5th. Starting with the line it's on. Five lines and spaces or five letter names. The number of half steps is 7. I'll leave it to you to count them. A 5th that sounds like this with 7 half steps is called a perfect fifth. Why perfect? Perhaps because it is the interval with the simplest ratio of three to two, outside the unison in the octave, remember our overtone series discussion several weeks ago. The perfect 5th has historically been considered the most stable interval. The most constant interval beside the octave. There are three other perfect intervals. The perfect unison, or first. The perfect octave or 8th. And the perfect 4th. The first two are easy to understand why they are considered perfect. The perfect 4th is less clear. The perfect 4th is considered to be a dissonant interval, so what's so perfect about it? Perhaps because when we turn a perfect 4th outside down, placing the bottom note on the top, it becomes a perfect 5th. C up to F is a perfect 4th, F up to C is a perfect 5th. This is called inverting the interval, which we'll talk about later. Notice that F and C are both notes in C major. This is a very handy way to figure out intervals. If you know your major keys, the tonic up to the 4th and 5th scale degrees is always perfect. In C major, C up to F is a perfect 4th. C up to G is a perfect 5th. In B flat major, which has two flats shown here, B flat and E flat, B flat up to E flat is a perfect 4th. And B flat up to F is a perfect 5th. We don't need to count the half steps. We just know. Now you try one. What is a perfect 5th above C sharp? We know it must be some kind of G since that's a 5th above C. C sharp major has seven sharps, or every note is sharp, so it must be a G sharp. Count the half steps to prove it to yourself. They must add up to seven. What is a perfect 4th above A flat? We know it must be some kind of D which is four letters names starting with A. Or four lines and spaces, starting with the space A is on above A flat. The key of A flat major has four flats. B, E, A and D flat. So the perfect 4th above A flat would be D flat. It should be five half steps. A flat to A. A to B flat. B flat to B. B to C and C to D flat. So five half steps. Here's a harder one. What is a perfect 5th above D sharp? Five letter names up from D, starting with D, is A. So it must be some kind of an A in order to be a 5th. But there's no key of D sharp major, so we can't directly use our major key method. But we can figure out a perfect 5th up from D natural using the key of D major, two sharps, which would be A natural. If we sharp both notes, D becomes D sharp and A becomes A sharp, so that must also be a perfect 5th. Notice that all perfect intervals always share the same accidentals. E to A and B. C# to F# and G#. D flat to G flat and A flat. This is actually the easiest way to figure out perfect 4ths and 5ths. There is one exception to this though. B to F And F to B aren't perfect. There that's the start on describing intervals more precisely, based on how they're notated and how they sound. In addition to the generic size of interval, which uses a number, we also talk about the interval's quality. So the exact description of an interval combines the two. A generic 5th which could have several possible sounds becomes an exact pair of notes spelled in a specific way when we caught a perfect 5th. We learned about perfect 4ths and 5ths, which are always within a key, one to four or five or DO to FA or so. Finally we learned a shortcut for spelling perfect intervals. They almost always, except when they involve B and F, share the same accidental. Sharp to sharp, flat to flat, natural to natural. In the next lecture, we'll look at the other intervals that occur within a key, 2nds, 3rds, 6ths, and 7ths. We'll also consider intervals between notes that aren't within a key. [MUSIC]
