diff --git a/RELEASE-NOTES-5.1.0b_RC1.html b/RELEASE-NOTES-5.1.0b_RC1.html
index 5bfe5b1d..cb94988e 100644
--- a/RELEASE-NOTES-5.1.0b_RC1.html
+++ b/RELEASE-NOTES-5.1.0b_RC1.html
@@ -176,6 +176,53 @@ order <strong>main, radar, MFD 1, MFD 2</strong>:</p>
 <p>Use <code>0</code> in any position to leave that one on automatic -- <code>-tmon 2,1,0,0</code> swaps only main and radar.
 <strong>Leave the switch off entirely and everything is detected automatically</strong>, which is correct on
 most pods.</p>
+<p><strong>Which positions apply depends on the MFD mode:</strong></p>
+<table>
+<thead>
+<tr>
+<th>Position</th>
+<th>Display</th>
+<th><code>-tmfds 4</code> (split)</th>
+<th><code>-tmfds 1</code> / <code>-tmfds 3</code> (spanned)</th>
+</tr>
+</thead>
+<tbody>
+<tr>
+<td>1st</td>
+<td>Main</td>
+<td>applies</td>
+<td>applies</td>
+</tr>
+<tr>
+<td>2nd</td>
+<td>Radar</td>
+<td>applies</td>
+<td>applies</td>
+</tr>
+<tr>
+<td>3rd</td>
+<td>MFD 1</td>
+<td>applies</td>
+<td>ignored</td>
+</tr>
+<tr>
+<td>4th</td>
+<td>MFD 2</td>
+<td>applies</td>
+<td>ignored</td>
+</tr>
+</tbody>
+</table>
+<p>In the spanned modes the first two positions work normally, so you can still correct main and
+radar. The 3rd and 4th positions are accepted but have no effect, because the spanned modes
+drive both MFDs from a single wide display rather than two separate ones.</p>
+<p><strong>The spanned MFD display itself cannot be assigned with <code>-tmon</code>.</strong> It is picked automatically
+as the first display that reports a 1280x480 mode -- a distinctive signature that a spanned
+MFD pair produces and ordinary monitors do not, so it normally lands on the right one. If you
+ever need to force it, ask and it can be added.</p>
+<p>If a display comes up in the wrong place, check <code>gos-displays.txt</code> next to the game (see
+section 8). It lists every display found, which role each was given, and whether each <code>-tmon</code>
+position was accepted or ignored.</p>
 <hr>
 <h2>5. 'Mechs and content</h2>
 <h3>Six rookie 'Mechs instead of four</h3>
@@ -301,7 +348,7 @@ start. Previously such failures were completely silent. Include this file with a
 </tr>
 <tr>
 <td><code>-tmon a,b,c,d</code></td>
-<td>Force monitor order: main, radar, MFD1, MFD2</td>
+<td>Force monitor order: main, radar, MFD1, MFD2. Positions 3 and 4 apply only to <code>-tmfds 4</code></td>
 <td>Automatic detection</td>
 </tr>
 <tr>
diff --git a/RELEASE-NOTES-5.1.0b_RC1.md b/RELEASE-NOTES-5.1.0b_RC1.md
index a68e3a7c..ca0a13d0 100644
--- a/RELEASE-NOTES-5.1.0b_RC1.md
+++ b/RELEASE-NOTES-5.1.0b_RC1.md
@@ -195,6 +195,28 @@ Use `0` in any position to leave that one on automatic -- `-tmon 2,1,0,0` swaps
 **Leave the switch off entirely and everything is detected automatically**, which is correct on
 most pods.
 
+**Which positions apply depends on the MFD mode:**
+
+| Position | Display | `-tmfds 4` (split) | `-tmfds 1` / `-tmfds 3` (spanned) |
+|---|---|---|---|
+| 1st | Main | applies | applies |
+| 2nd | Radar | applies | applies |
+| 3rd | MFD 1 | applies | ignored |
+| 4th | MFD 2 | applies | ignored |
+
+In the spanned modes the first two positions work normally, so you can still correct main and
+radar. The 3rd and 4th positions are accepted but have no effect, because the spanned modes
+drive both MFDs from a single wide display rather than two separate ones.
+
+**The spanned MFD display itself cannot be assigned with `-tmon`.** It is picked automatically
+as the first display that reports a 1280x480 mode -- a distinctive signature that a spanned
+MFD pair produces and ordinary monitors do not, so it normally lands on the right one. If you
+ever need to force it, ask and it can be added.
+
+If a display comes up in the wrong place, check `gos-displays.txt` next to the game (see
+section 8). It lists every display found, which role each was given, and whether each `-tmon`
+position was accepted or ignored.
+
 ---
 
 ## 5. 'Mechs and content
@@ -346,7 +368,7 @@ start. Previously such failures were completely silent. Include this file with a
 | Switch | Purpose | If you leave it off |
 |---|---|---|
 | `-tmfds 4` | Two separate 640x480 MFD monitors | Existing MFD mode unchanged |
-| `-tmon a,b,c,d` | Force monitor order: main, radar, MFD1, MFD2 | Automatic detection |
+| `-tmon a,b,c,d` | Force monitor order: main, radar, MFD1, MFD2. Positions 3 and 4 apply only to `-tmfds 4` | Automatic detection |
 | `-tbaud <rate>` | COM1 speed for replica RIO boards | Original board speed |
 | `-fps` | Write a performance log | No log, no overhead |
 | `-help` | Show the full switch reference and exit | Game starts normally |