In this lesson, we will consider intervals that have no accidentals–natural intervals (located on the white keys of the piano). It is easy to identify natural intervals based on the number of natural half steps they contain. Remember that natural half steps occur between EF and BC.
The following examples present some natural intervals. Each interval contains either 0, 1, or 2 natural half steps. Study these examples carefully and see if you can determine how many natural half steps are in each.
As you can see in the chart below, the fewer the natural half steps, the larger the interval. Vice versa, the more natural half steps contained in an interval, the smaller the interval. For the smaller intervals (2nds and 3rds), there will be either 0 natural half steps (major 2/3) or 1 natural half step (minor 2/3). For the larger intervals (6ths and 7ths), there will be either 1 natural half step (major 6/7) or 2 natural half steps (minor 6/7).

Notice on the keyboard that there is only one fourth (FB) which contains no natural half steps and only one fifth (BF) which contains two natural half steps. The rules for fourths and fifths can thus be stated:

The following method is recommended for recognizing intervals without accidentals:
 Determine the number size of the interval.
 Determine the number of natural half steps that occur within the interval.
 Label the interval quality based on its number size and the number of natural half steps it has (e.g., a 6th with one natural half step is major).
Let’s revisit the four intervals from the top of this page. What is the size and quality of each?
Quality of Natural Intervals
Online drills:
Identification of natural intervals
MTSB Lesson 5, practice identifying and notating intervals (number size only)
MTSB Lesson 5, practicing indentifying natural intervals by number size and quality