Help with minor/dim intervals

[thorrian]

Okay, so I am really confused about one of the intervals my book is teaching me.  It says if you take a M3 interval and lower it one half step it becomes a m3, that makes sense to me.  But then it says if you lower the m3 another half step it becomes diminished.  I don't understand because I thought that would just make it a M2 interval.  This has me all hung up on trying to study these intervals.  Can anyone please clarify this for me? Please? =D

[apyrase]

So the way I understand it, if you start at a minor or a perfect interval and lower it by a half step, it becomes a diminished interval. For example, C# to E# is a major third interval, from C# to E is a minor third, and from C# to E♭ is a diminished third. Also, the tritone is a diminished fifth. The tritone is also an augmented fourth and C# to Ex is an augmented third.

Hope this helps

(Thanks to perfect pitch for pointing out my typo)

[stevebob]

Okay, so I am really confused about one of the intervals my book is teaching me.  It says if you take a M3 interval and lower it one half step it becomes a m3, that makes sense to me.  But then it says if you lower the m3 another half step it becomes diminished.  I don't understand because I thought that would just make it a M2 interval.  This has me all hung up on trying to study these intervals.  Can anyone please clarify this for me? Please? =D

Consider how sharps and flats function—and the reason why F♯ and G♭, for example, aren't the same.  They're considered enharmonic equivalents and are played on the same key, but they're different because F and G are different notes of the scale.

Now think of double-sharps and double-flats, and consider, for example, the relationship between D♮ and E♭♭.  They're enharmonically equivalent and are played on the same key, but they're different because D and E are different notes of the scale.

Now consider C, D and E.  They're different notes—i.e., different letter names, different degrees of the scale.  The interval between C-D is a second and the interval between C-E is a third—no matter whether they're sharps, flats or naturals—because D is a second away from C, and E is a third away from C.

So ... while the interval between C♮ and D♮ is a major second, the interval between C and E♭♭—though it sounds the same, and is played on the same keys—is a diminished third.  It's not the same because D and E are different notes of the scale.

If this doesn't make sense, then think of a C diminished triad. Because a triad is comprised of a root, a third above and a fifth above, it must be written C-E♭-G♭.  It can't be written C-D♯-F♯ (even though those notes are enharmonically equivalent) because D and F are a second and a fourth above C instead of the third and fifth that are required.  Likewise, a C diminished seventh chord is written C-E♭-G♭-B♭♭ and not C-E♭-G♭-A because the note that's a seventh away from C is B, not A.

[perfect_pitch]

Also, the tritone is a diminished fifth. The tritone is also an augmented fifth

ERM... don't you mean:

Also, the tritone is a diminished fifth. The tritone is also an augmented fourth?