Understanding Intervals in Music Theory

Understanding and knowing how to interpret intervals can really benefit a musician's performing abilities because they are key drivers of musical momentum. How so, you may ask?

What’s In an Interval?

An interval is a measurement of distance between two pitches in a musical space, whether the two pitches are playing simultaneously in harmony or sequentially as part of a melody. Either way, the greater the distance between pitches, the more energy or tension a listener feels.

We can measure (or “count”) an interval's size in two ways:

• Counting the number of half steps (semitones) between pitches on an instrument such as a keyboard
• Counting the number of staff positions between pitches on a musical staff

Counting Steps

A half step (or one semitone) is the closest distance between two pitches in a Western chromatic scale. On a keyboard, there is exactly one half step between any two adjacent keys.

Listen to a half step:

A whole step is the distance of two half steps (semitones) between pitches in a Western chromatic scale, such as C to D.

Counting Staff Positions

Each line or space on a staff is a position that we count when measuring the distance between pitches. So, in counting from one note to the next, the note we begin with is counted as 1. Then, we count to the next line or space as 2, then the next line or space as 3, and so on.

When we do this we get the following musical measurements:

• A unison occurs when two pitches "sit" on the same line or space
• A second occurs when pitches are adjacent (or next to each other) on the staff (e.g., a line next to a space)
• A third occurs when we skip over a staff position from line to line or space to space
  • For example, starting from a note that is on a space, we count to the adjacent line to create a second, or to the next space to make a third
  • OR, if we are starting from a note that is on a line, we count to the next adjacent space to create a second or to the next line to make a third.

Remember, always start counting from the lowest note!

Interval Quality

In addition to measuring interval size, musicians also assess the quality of a musical interval. Quality is a way of categorizing different types of interval sounds. Some intervals are more pure or "hollow sounding," while other intervals are more rich in "sonic color."

There are two general categories of intervallic quality:

• perfect intervals (these are the "hollow sounding" intervals)
• imperfect intervals (these are the intervals that possess a little more variety of harmonic color)

Perfect Intervals

The perfect intervals are:

• unisons (or zero semitones)
• 4ths (or five semitones)
• 5ths (or seven semitones)
• octaves (or 12 semitones)

Perfect intervals may be stretched or compressed. If we increase a perfect interval by a semitone, then we say that interval has been augmented. If we decrease a perfect interval by a semitone, then we say that the interval has been diminished.

So...unisons, 4ths, 5ths, and octaves may be:

• augmented (+)
• perfect (P)
• diminished (o)

Imperfect Intervals

The imperfect intervals are:

• 2nds (either one or two semitones)
• 3rds (either three or four semitones)
• 6ths (either eight or nine semitones)
• 7ths (either 10 or 11 semitones)

Unlike perfect intervals, you’ll notice that we have options for each size. Imperfect intervals can be either major or minor. So, more specifically:

• minor 2nd (m2) = one semitone
• major 2nd (M2) = two semitones
• minor 3rd (m3) = three semitones
• major 3rd (M3) = four semitones
• minor 6th (m6) = eight semitones
• major 6th (M6) = nine semitones
• minor 7th (m7) = 10 semitones
• major 7th (M7) = 11 semitones

Imperfect intervals may also be stretched or compressed. If we increase a major interval by a semitone, then we say that interval has been augmented. If we decrease a minor interval by a semitone, then we say that the interval has been diminished.

• So...2nds, 3rds, 6ths, and 7ths may be:
  • augmented (+)
  • major (M)
  • minor (m)
  • diminished (o)

Consonance and Dissonance

The quality of an interval matters, because when two notes are sounded simultaneously, we interpret the sound as having a kind of motion or lack thereof. Some intervals played simultaneously sound restful; others feel active, as if they "want" to go forward. We call this relative stability or instability consonance and dissonance, respectively.

• Consonance: The impression of stability and repose experienced by a listener when certain combinations of tones or notes are sounded together.
• Dissonance: The impression of tension or instability experienced by a listener when certain combinations of tones or notes are sounded together.

Measures of Stability