diff --git a/benches/grid_projection.rs b/benches/grid_projection.rs
index 946610e4..83e73069 100644
--- a/benches/grid_projection.rs
+++ b/benches/grid_projection.rs
@@ -62,7 +62,8 @@ fn grouped_app(days: usize, per_group: usize, products: usize, measures: usize)
     wb.add_category("Product").unwrap();
     for d in 0..days {
         let group = format!("G{}", d / per_group);
-        wb.add_item_in_group("Day", format!("Day{d}"), group).unwrap();
+        wb.add_item_in_group("Day", format!("Day{d}"), group)
+            .unwrap();
     }
     for p in 0..products {
         wb.add_item("Product", format!("Product{p}")).unwrap();
diff --git a/context/code/command.md b/context/code/command.md
index 6ec985ca..8b05d17a 100644
--- a/context/code/command.md
+++ b/context/code/command.md
@@ -1805,11 +1805,44 @@ one `CellKey` per member, the cross product of every spanned category's covered
 items (3,000 keys for an 18k column), and looked each one up. Now
 `group_expansion` returns the expansion as data: a `base` key and the covered
 items of each expanded category. `fold_stored` hands that to
-`DataStore::spanned_values`, which narrows by `base` through the index once and
-filters each candidate by membership. In `Exact` mode a candidate must also
-carry no coordinate beyond those, which is what `get(member)` answered. Only a
-formula still enumerates members (`contexts_of`), since its references are
-evaluated per context.
+`DataStore::spanned_values`, which narrows through the index and filters each
+candidate by membership. In `Exact` mode a candidate must also carry no
+coordinate beyond those, which is what `get(member)` answered. Only a formula
+still enumerates members (`contexts_of`), since its references are evaluated
+per context.
+
+**How it narrows is a choice, and the first version got it wrong
+(improvise-jq0m).** Narrowing by `base` alone is right for a column total, which
+spans *every* item of each row category. It is wrong for a collapsed group,
+which spans two or seven items of a category with hundreds: `base` drops the
+grouped category entirely, so every fold cell scanned every record of the
+column, and a grid of small groups went quadratic. The review measured 1000
+days in pairs at 11–15.7 s a keypress, against 0.44–0.51 s before #239. So
+`spanned_positions` now prices both narrowings off the index's own set sizes
+and takes the cheaper: once by `base`, or once per covered item of the span
+with the fewest items, each intersected with `base`. The per-item positions are
+disjoint (a record has one item per category) and are sorted afterwards, so
+both paths return the store's order. A `base` naming a name the store never
+interned matches nothing on both paths.
+
+The bench now carries the grouped cases, which it could not see before: Day in
+collapsed groups on Row, with Product, and `_Measure` on Column. The machine is
+an Apple M4 Max (16 cores, 64 GB) at load average 15–63. The figures are
+keypress (`whole-grid + col_widths`), min-of-N, from two alternating rounds,
+the second reversed. Each cell gives both rounds:
+
+| case | before #239 (`5932ad74`) | `f668ddc9` | jq0m | jq0m ÷ before |
+|---|---|---|---|---|
+| 4.8k dense | 33.9 / 39.1 ms | 40.7 / 40.2 ms | 36.3 / 33.3 ms | 1.07 / 0.85 |
+| 18k dense | 237 / 239 ms | 238 / 232 ms | 196 / 211 ms | 0.83 / 0.88 |
+| 90k dense | 3.04 / 3.46 s | 3.24 / 2.70 s | 2.68 / 2.84 s | 0.88 / 0.82 |
+| 364 days in weeks | 483 / 415 ms | 1.42 / 1.18 s | 356 / 399 ms | 0.74 / 0.96 |
+| 364 days in pairs | 715 / 539 ms | 5.02 / 4.62 s | 471 / 518 ms | 0.66 / 0.96 |
+| 1000 days in pairs | 406 / 399 ms | 10.8 / 20.2 s | 352 / 364 ms | 0.87 / 0.91 |
+
+The grouped cases now come in **below** the pre-#239 figures. The old
+per-member path looked up each member through a full `CellKey`, and the
+per-item narrowing intersects index sets directly.
 
 Two levers were **not** taken. Combining the walk's row values for decomposable
 aggregators was rejected under §5 as a second derivation of the total. It is
diff --git a/context/test-evidence/core.md b/context/test-evidence/core.md
index 2a30c19a..1df4babf 100644
--- a/context/test-evidence/core.md
+++ b/context/test-evidence/core.md
@@ -1537,3 +1537,30 @@ Three faults were injected, each run against the full `improvise-core` and
   (collapsed-group, hidden-row and CSV-agreement tests).
 - A category the store never interned is skipped instead of matching nothing.
   **Only this test fails.**
+
+### a_span_finds_what_its_members_find_in_store_order
+
+improvise-jq0m. `spanned_values` now chooses between two narrowings, and a
+fixed example can only ever take one of them. This property builds random
+stores over `R`, `P` and `M`. Some records lack `R`, some carry an extra `X`,
+and `r6` is a covered item the store never interned. It spans `R` and sometimes
+`P`, in both modes, and compares the result with the per-member definition,
+**in store order**: the values the members' own lookups find, read back in
+`iter_cells` order. Store and span sizes vary across cases, so both narrowings
+are taken.
+
+Three faults were injected, each run against the full `improvise-core` and
+`improvise-command` suites:
+
+- The per-item narrowing drops the base, so it keeps each item's pair alone.
+  This test and `a_spanned_scan_finds_what_a_lookup_per_member_finds` fail.
+  **Nothing in either crate's grid or command tests does.** Those fixtures never
+  hold a second value of the base's coordinate within a small span.
+- The base narrowing drops the base. This test fails, along with eight
+  fold and totals tests.
+- The per-item positions are not sorted after they are merged. **Only this
+  test fails.** Values stay correct and only their order moves, which changes
+  a Sum or Avg in the last bits.
+
+The regression itself is a cost, which no test here can see. The evidence for
+it is the `grid_projection` bench's grouped cases, added for this ticket.
diff --git a/crates/improvise-core/src/model/cell.rs b/crates/improvise-core/src/model/cell.rs
index 19254613..dbae8faa 100644
--- a/crates/improvise-core/src/model/cell.rs
+++ b/crates/improvise-core/src/model/cell.rs
@@ -484,7 +484,11 @@ impl DataStore {
             .unwrap_or_default()
     }
 
-    fn narrowed(&self, base: &[(Symbol, Symbol)], wanted: &[(Symbol, HashSet<Symbol>)]) -> Vec<usize> {
+    fn narrowed(
+        &self,
+        base: &[(Symbol, Symbol)],
+        wanted: &[(Symbol, HashSet<Symbol>)],
+    ) -> Vec<usize> {
         let with = |cat: Symbol, item: Symbol| [base, &[(cat, item)]].concat();
         let per_item: Vec<Vec<(Symbol, Symbol)>> = wanted
             .iter()