diff --git a/doc/4-instrument/fm-esfm.md b/doc/4-instrument/fm-esfm.md
index 147d3f948..fa326fc17 100644
--- a/doc/4-instrument/fm-esfm.md
+++ b/doc/4-instrument/fm-esfm.md
@@ -24,6 +24,7 @@ these apply to the instrument as a whole:
   - these are normally used for the drum channels in ESFM's OPL3 compatibility mode.
     - however, in ESFM, operator 4 can be modulated by operator 3, whereas in OPL3 drum mode the rhythm channels cannot be modulated.
   - __note__: usage of noise mode "Top" is discouraged for now as it is not properly emulated yet, and results may change when the emulation gets fixed in the future.
+- **Octave**: sets the "block" of the frequency register, which affects note range and precision.
 - **operator routing preview**: shows how operators are connected with each other and with the audio output (at the bottom).
   - left-click pops up a small "operators changes with volume?" dialog where each operator can be toggled to scale with volume level.
   - right-click switches to a preview display of the waveform generated on a new note:
diff --git a/doc/4-instrument/fm-opl.md b/doc/4-instrument/fm-opl.md
index 0f16346ce..4a4f7e5ec 100644
--- a/doc/4-instrument/fm-opl.md
+++ b/doc/4-instrument/fm-opl.md
@@ -23,6 +23,7 @@ these apply to the instrument as a whole:
     - right-click returns to algorithm view.
 - **Feedback (FB)**: determines how many times operator 1 returns its output to itself (0 to 7).
 
+- **Octave**: sets the "block" of the frequency register, which affects note range and precision.
 - **4-op**: enables 4-operator FM instrument editor mode (only on OPL3).
 - **Drums**: enables OPL drum mode editor.
 
diff --git a/doc/4-instrument/fm-opll.md b/doc/4-instrument/fm-opll.md
index 902c65812..141ddfb53 100644
--- a/doc/4-instrument/fm-opll.md
+++ b/doc/4-instrument/fm-opll.md
@@ -18,8 +18,10 @@ but don't worry! there also are 15 preset patches that you may select at any tim
 these apply to the instrument as a whole:
 - **Feedback (FB)**: determines how many times operator 1 returns its output to itself (0 to 7).
 - **Sustain (SUS)**: enables the sustain flag (sets the release rate to 5).
-- **DC (half-sine carrier)**: sets the waveform produced by carrier operator to half-sine.
-- **DM (half-sine modulator)**: sets the waveform produced by modulator operator to half-sine.
+
+- **Octave**: sets the "block" of the frequency register, which affects note range and precision.
+- **OP2 Half Sine (DC)**: sets the waveform produced by carrier operator to half-sine.
+- **OP1 Half Sine (DM)**: sets the waveform produced by modulator operator to half-sine.
 - **preset dropdown**: selects OPLL preset instrument.
   - this is the selector for the preset patches I mentioned before.
   - once a preset patch is selected, only the volume is configurable.
diff --git a/doc/4-instrument/fm-opn.md b/doc/4-instrument/fm-opn.md
index 9a287c89f..6c0c3b70f 100644
--- a/doc/4-instrument/fm-opn.md
+++ b/doc/4-instrument/fm-opn.md
@@ -28,6 +28,8 @@ these apply to the instrument as a whole:
   - only applies to operators which have AM turned on.
   - does not apply to YM2203.
 
+- **Octave**: sets the "block" of the frequency register, which affects note range and precision.
+
 these apply to each operator:
 - the crossed-arrows button can be dragged to rearrange operators.
 - the **OP1**, **OP2**, **OP3**, and **OP4** buttons enable or disable those operators.