diff --git a/doc/4-instrument/fm-opll.md b/doc/4-instrument/fm-opll.md
index 87eac0253..6f7fc3a9b 100644
--- a/doc/4-instrument/fm-opll.md
+++ b/doc/4-instrument/fm-opll.md
@@ -43,7 +43,7 @@ these apply to each operator:
 ![FM ADSR chart](FM-ADSRchart.png)
 
 - **Envelope Scale (KSR)**: also known as "Key Scale". determines the degree to which the envelope execution speed increases according to the pitch.
-- **Frequency Multiplier (MULT)**: sets the coarse pitch offset in relation to the note (0 to 15). 0 is -1 octave, 1 is 0 octaves, 2 is 1 octave, 3 is 1 octave 7 semitones, and so on.
+- **Frequency Multiplier (MULT)**: sets the coarse pitch offset in relation to the note (0 to 15). The values follow the harmonic scale. For example, 0 is -1 octave, 1 is 0 octaves, 2 is 1 octave, 3 is 1 octave 7 semitones, and so on.
   - note that values 11, 13 and 14 behave as 10, 12 and 12 respectively.
 - **Vibrato (VIB)**: makes the operator affected by LFO vibrato.