diff --git a/src/components/DatumPanel.vue b/src/components/DatumPanel.vue
index 0778e47..809408e 100644
--- a/src/components/DatumPanel.vue
+++ b/src/components/DatumPanel.vue
@@ -96,6 +96,7 @@ function axisBadge(datum: Datum): string | null {
     if (datum.type === "rectangle" && datum.isAxisReference) return "axis"
     if (datum.type === "line" && datum.axisRole === "x") return "+x"
     if (datum.type === "line" && datum.axisRole === "y") return "+y"
+    if (datum.type === "ellipse" && datum.isPrimary) return "primary"
     return null
 }
 </script>
@@ -407,6 +408,35 @@ function axisBadge(datum: Datum): string | null {
                             </button>
                         </div>
                     </div>
+                    <div
+                        v-else-if="datum.type === 'ellipse'"
+                        class="flex items-center justify-between"
+                    >
+                        <Label class="text-xs">Primary reference</Label>
+                        <label
+                            class="flex cursor-pointer items-center gap-1.5"
+                            @click.stop
+                        >
+                            <input
+                                type="checkbox"
+                                class="accent-primary"
+                                :checked="datum.isPrimary ?? false"
+                                @change="
+                                    (e) =>
+                                        store.setAxisRole(
+                                            datum.id,
+                                            (e.target as HTMLInputElement)
+                                                .checked
+                                                ? 'ellipse'
+                                                : null,
+                                        )
+                                "
+                            />
+                            <span class="text-xs text-muted-foreground"
+                                >Use</span
+                            >
+                        </label>
+                    </div>
 
                     <!-- Confidence -->
                     <div>
diff --git a/src/lib/solver.ts b/src/lib/solver.ts
index 999d1c5..2fa88bb 100644
--- a/src/lib/solver.ts
+++ b/src/lib/solver.ts
@@ -268,7 +268,10 @@ type Primary =
 function pickPrimary(datums: Datum[]): Primary {
     if (datums.length === 0) throw new Error("No datums provided.")
 
-    // User-flagged world-axis reference wins regardless of type priority.
+    // User-flagged primary wins regardless of type priority. Rect's
+    // `isAxisReference` and line's `axisRole` carry axis semantics on top
+    // of "primary"; ellipse's `isPrimary` is a pure primary flag (ellipses
+    // don't define axis directions on their own).
     for (const d of datums) {
         if (d.type === "rectangle" && d.isAxisReference) {
             return { kind: "rect", datum: d }
@@ -276,6 +279,9 @@ function pickPrimary(datums: Datum[]): Primary {
         if (d.type === "line" && d.axisRole) {
             return { kind: "line", datum: d }
         }
+        if (d.type === "ellipse" && d.isPrimary) {
+            return { kind: "ellipse", datum: d }
+        }
     }
 
     const typeRank = (d: Datum): number =>
diff --git a/src/stores/app.ts b/src/stores/app.ts
index 790fea7..9037130 100644
--- a/src/stores/app.ts
+++ b/src/stores/app.ts
@@ -103,14 +103,15 @@ export const useAppStore = defineStore("app", () => {
         }
     }
 
-    /** Set (or clear) the world-axis role on a datum, enforcing that at
+    /** Set (or clear) the gauge-primary role on a datum, enforcing that at
      *  most one datum holds the role at a time.
-     *  `role`: "rect"  → rectangle.isAxisReference = true
-     *          "x"/"y" → line.axisRole = "x"|"y"
-     *          null    → clear the role on `id` (no-op if it wasn't set). */
+     *  `role`: "rect"    → rectangle.isAxisReference = true
+     *          "x"/"y"   → line.axisRole = "x"|"y"
+     *          "ellipse" → ellipse.isPrimary = true
+     *          null      → clear the role on `id` (no-op if it wasn't set). */
     function setAxisRole(
         id: string,
-        role: "rect" | "x" | "y" | null,
+        role: "rect" | "x" | "y" | "ellipse" | null,
     ) {
         // Clear any existing flag on other datums.
         for (let i = 0; i < datums.value.length; i++) {
@@ -120,6 +121,8 @@ export const useAppStore = defineStore("app", () => {
                 datums.value[i] = { ...d, isAxisReference: false }
             } else if (d.type === "line" && d.axisRole) {
                 datums.value[i] = { ...d, axisRole: null }
+            } else if (d.type === "ellipse" && d.isPrimary) {
+                datums.value[i] = { ...d, isPrimary: false }
             }
         }
         const idx = datums.value.findIndex((d) => d.id === id)
@@ -131,6 +134,8 @@ export const useAppStore = defineStore("app", () => {
                 datums.value[idx] = { ...target, isAxisReference: false }
             } else if (target.type === "line") {
                 datums.value[idx] = { ...target, axisRole: null }
+            } else {
+                datums.value[idx] = { ...target, isPrimary: false }
             }
             return
         }
@@ -138,6 +143,8 @@ export const useAppStore = defineStore("app", () => {
             datums.value[idx] = { ...target, isAxisReference: true }
         } else if ((role === "x" || role === "y") && target.type === "line") {
             datums.value[idx] = { ...target, axisRole: role }
+        } else if (role === "ellipse" && target.type === "ellipse") {
+            datums.value[idx] = { ...target, isPrimary: true }
         }
     }
 
diff --git a/src/types/index.ts b/src/types/index.ts
index 2473a78..883b86d 100644
--- a/src/types/index.ts
+++ b/src/types/index.ts
@@ -49,6 +49,10 @@ export interface EllipseDatum {
     diameterMm: number
     confidence: 1 | 2 | 3 | 4 | 5
     label: string
+    /** When true, this ellipse is the gauge primary — overrides the
+     *  type-rank auto-pick. Mutually exclusive with `RectDatum.isAxisReference`
+     *  and `LineDatum.axisRole`; setting any of those clears the others. */
+    isPrimary?: boolean
 }
 
 export type Datum = RectDatum | LineDatum | EllipseDatum