What are the Prime 5ths in piano tuning?

Prime 5ths and Prime 4ths within the Prime Octave

Prime 5ths

The Prime 5ths are specific intervals contained within the Prime Octave, defined by the reference notes A3 and A4. There are two main Prime 5ths:

• Lower Prime 5th: A3/E4
• Upper Prime 5th: D4/A4

These Prime 5ths serve an important role in determining the width of the Prime Octave.
Utilizing the widths of the Prime 5ths is a new approach to defining the width of the Prime octave.

Prime 4ths

The Prime 4ths are ‘resultant’ intervals.  The Prime 4ths are:

• Lower Prime 4th: A3/D4
• Upper Prime 4th: E4/A4

The upper and lower Prime 4ths are considered ‘resultant’ intervals that emerge naturally when balancing the Prime 5ths. Although these intervals are rarely measured in practice, they provide valuable aural checks for ensuring that the 5ths are properly balanced.