diff --git a/public_html/README.md b/public_html/README.md
index 75fa9f6..945d273 100644
--- a/public_html/README.md
+++ b/public_html/README.md
@@ -60,6 +60,13 @@ except installation and the service worker.
   it paused — everything in there resets the clocks anyway — while the score
   sheet picks the turn back up when you close it, so glancing at the score
   doesn't cost you a tap on play.
+- **Settings are only reachable before a match starts.** Every control in that
+  sheet resets both clocks, so the gear disappears the moment the first tap
+  starts the clock and comes back after a reset. Its space in the bar is kept
+  empty, so nothing else shifts. That means a mistake in the time or delay can
+  only be fixed by resetting and starting the match again — deliberately, since
+  the alternative is wiping two live clocks by accident. Correcting a clock
+  mid-match is what the score sheet is for.
 
 Backgammon clocks pause between games, and after a cocked die the delay is
 normally restarted — use pause and reset for those.
diff --git a/public_html/index.html b/public_html/index.html
index b231531..0244774 100644
--- a/public_html/index.html
+++ b/public_html/index.html
@@ -165,6 +165,10 @@
   .btn svg{width:clamp(23px,6.4vw,32px);height:auto;display:block}
   .btn svg[hidden]{display:none}
   .btn[hidden]{display:none}      /* .btn's own display would otherwise win */
+  /* not [hidden]: that would collapse the slot and shift the whole bar. This
+     keeps the space, and still drops the button out of the tab order, the
+     accessibility tree and the path of a stray tap. */
+  .btn--vacant{visibility:hidden}
   .btn:focus-visible{outline:2px solid var(--icon-hot);outline-offset:-6px;border-radius:8px}
 
   /* reset armed for a second tap — the ring unwinds over the same 2s as the timeout */
@@ -591,6 +595,7 @@
   var btnSound = document.getElementById("sound");
   var btnReset = document.getElementById("reset");
   var btnScore = document.getElementById("score");
+  var btnSettings = document.getElementById("settings");
   var sheet = document.getElementById("sheet");
   var scoreSheet = document.getElementById("scoresheet");
   var scrim = document.getElementById("scrim");
@@ -681,6 +686,18 @@
     return Math.max(0, st.reserve[i] - consumed(i));
   }
 
+  /* Every timer control in the settings resets both clocks, so the sheet is only
+     safe to reach in exactly the state resetAll() leaves behind. Checking the
+     phase alone would not do: a score change runs newGame(), which returns to
+     IDLE waiting for the next game's first tap, with the match still very much
+     on. The score and reserve checks are what cover that. */
+  function matchPristine(){
+    return st.phase === IDLE && st.active === -1 &&
+           st.moves[0] === 0 && st.moves[1] === 0 &&
+           st.score[0] === 0 && st.score[1] === 0 &&
+           st.reserve[0] === matchTime() && st.reserve[1] === matchTime();
+  }
+
   /* ---- write only on change ----
      render() runs 60 times a second but the digits turn over once a second, and
      .time is a ~190px glyph run: rewriting its text node every frame forces a
@@ -735,6 +752,10 @@
         (st.phase === FLAG && i === st.active) ? "flagged"
         : (i === st.active && st.phase !== IDLE) ? "active" : "idle");
     }
+
+    // here rather than in the mutators, so it can never go stale; the
+    // two-argument toggle leaves the DOM alone when the state already matches
+    btnSettings.classList.toggle("btn--vacant", !matchPristine());
   }
 
   function loop(){ render(); requestAnimationFrame(loop); }
@@ -1026,7 +1047,10 @@
   }
   function closeSheet(){ openSheet(null); }
 
-  document.getElementById("settings").addEventListener("click", function(){
+  btnSettings.addEventListener("click", function(){
+    // the CSS only hides the button; the rule itself lives here, so no route in
+    // can reach the timer controls once the match is under way
+    if(!matchPristine()) return;
     pauseNow();                 // nobody's clock runs while a sheet is open
     disarm();
     paintSettings();
diff --git a/public_html/sw.js b/public_html/sw.js
index 61214cc..ddf8fef 100644
--- a/public_html/sw.js
+++ b/public_html/sw.js
@@ -1,5 +1,5 @@
 /* Cache-first: once installed the clock never touches the network again. */
-const CACHE = "bgclock-v7";
+const CACHE = "bgclock-v8";
 const FILES = [
   "./",
   "./index.html",