diff --git a/CHANGELOG.d/2.12.22-leftover-residual-disclosure.md b/CHANGELOG.d/2.12.22-leftover-residual-disclosure.md
new file mode 100644
index 000000000..cd9b4d2d2
--- /dev/null
+++ b/CHANGELOG.d/2.12.22-leftover-residual-disclosure.md
@@ -0,0 +1,7 @@
+## 2.12.22 — Leftover residual disclosure
+
+- Name signed leftover residual `R = Y − E[Y|θ, item]` on leftover
+  post–criterion pair rows (ADR 0178). After `make seed`, closest and
+  farthest leftover pairs sit above the member list with `R` next to
+  leftover-map distance `d`; click opens that post. A non-finite
+  residual is an em dash, never a fabricated leftover score.
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 6dbd797ef..f31b70545 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -213,6 +213,15 @@ All notable changes to this project are documented here. Format follows
   shell. The production frontend build type-checks again.
 
 
+## [2.12.22] - 2026-08-24
+
+### Added
+
+- Period leftover pair rows now name signed leftover residual
+  `R = Y − E[Y|θ, item]` next to leftover-map distance `d`, then open
+  that post (Jeon et al., 2021, eq. 3; ADR 0178). A non-finite residual
+  is an em dash rather than a fabricated leftover score.
+
 ## [2.12.21] - 2026-08-24
 
 ### Added
diff --git a/docs/adr/0178-leftover-residual-disclosure.md b/docs/adr/0178-leftover-residual-disclosure.md
new file mode 100644
index 000000000..0b1fe52a6
--- /dev/null
+++ b/docs/adr/0178-leftover-residual-disclosure.md
@@ -0,0 +1,67 @@
+# ADR 0178 — Disclose leftover residual on period-report pair rows
+
+**Decision status:** Accepted
+**Date:** 2026-08-24
+
+Amends [ADR 0049](0049-leftover-pair-report-ui.md).
+
+## Context
+
+ADR 0048 already persists leftover-map distance and leftover residual
+`R = Y − E[Y|θ, item]` on `report_leftover_pair`. ADR 0049 already
+renders closest and farthest pairs above the member list and opens the
+named post. The pair button showed only `d`, so a buyer could not tell
+a large leftover response from a merely distant map pair.
+
+Jeon et al. (2021, eq. 3) leftover interaction is
+`−γ‖ξ_p − ζ_i‖`. Distance is that map gap. Residual is the observed
+leftover *after IRT main effects* that entered the biplot. They are
+different quantities. Hiding residual would keep the persisted column
+as an unpublished measurement.
+
+This increment does not persist leftover-map coordinates, does not name
+observed `Y` / expected `E`, does not name leftover-map rank, and does
+not land Post quality on the leftover criterion. No schema change:
+`leftover_residual` already exists from ADR 0048 / migration `0012`.
+
+The unprotected-stack ADR for the same buyer fact was 0162. This
+protected-main reconstruction uses **0178** so it does not collide with
+two-axis leftover-map distance (0166), leftover coverage (0168),
+leftover-map axis share (0148), leftover-map rank (0172), leftover
+observed Y / expected E (0177), or analysis-run status same clock
+(0171).
+
+## Decision
+
+Each leftover pair button shows signed leftover residual `R` with two
+decimal places next to leftover-map distance `d`. Next action: leftover
+residual `R` after IRT main effects; open this post to read the named
+criterion. A non-finite residual renders an em dash rather than a
+fabricated leftover score. Click still uses the same post-open handler
+as ADR 0049.
+
+## Consequences
+
+`GET /api/reports/{grouping}/{period}` already returns
+`leftover_residual`. The frontend now names that value. After
+`make seed`, closest and farthest leftover pairs sit above the member
+list with `R` next to `d`; click opens that post.
+
+## Related
+
+Independent of leftover interaction-map persistence, leftover-criterion
+evaluation landing, leftover-map complete-case coverage, leftover-map
+axis share, leftover pairs on the grouping comparison strip, two-axis
+leftover-map distance, leftover observed `Y` / expected `E`, and
+leftover-map rank.
+
+## References
+
+Gabriel, K. R. (1971). The biplot graphic display of matrices with
+application to principal component analysis. *Biometrika, 58*(3),
+453–467. https://doi.org/10.1093/biomet/58.3.453
+
+Jeon, M., Jin, I. H., Schweinberger, M., & Baugh, S. (2021). Mapping
+unobserved item–respondent interactions: A latent space item response
+model with interaction map. *Psychometrika, 86*(2), 378–403.
+https://doi.org/10.1007/s11336-021-09762-5
diff --git a/frontend/src/leftoverResidual.test.ts b/frontend/src/leftoverResidual.test.ts
new file mode 100644
index 000000000..bcdb831c3
--- /dev/null
+++ b/frontend/src/leftoverResidual.test.ts
@@ -0,0 +1,11 @@
+import { describe, expect, it } from "vitest";
+import { formatLeftoverResidual } from "./leftoverResidual";
+
+describe("formatLeftoverResidual", () => {
+  it("keeps a signed residual without inventing a leftover score", () => {
+    expect(formatLeftoverResidual(0.4)).toBe("+0.40");
+    expect(formatLeftoverResidual(-1.1)).toBe("\u22121.10");
+    expect(formatLeftoverResidual(0)).toBe("0.00");
+    expect(formatLeftoverResidual(Number.NaN)).toBe("—");
+  });
+});
diff --git a/frontend/src/leftoverResidual.ts b/frontend/src/leftoverResidual.ts
new file mode 100644
index 000000000..7795ee43d
--- /dev/null
+++ b/frontend/src/leftoverResidual.ts
@@ -0,0 +1,18 @@
+/** Signed leftover residual ``R = Y − E[Y|θ, item]`` after IRT main effects. */
+
+export const LEFTOVER_RESIDUAL_ACTION =
+  "Leftover residual R {residual} after IRT main effects. Open this post to read {criterion}.";
+
+export function formatLeftoverResidual(value: number): string {
+  if (!Number.isFinite(value)) {
+    return "—";
+  }
+  const magnitude = Math.abs(value).toFixed(2);
+  if (value > 0) {
+    return `+${magnitude}`;
+  }
+  if (value < 0) {
+    return `\u2212${magnitude}`;
+  }
+  return magnitude;
+}
diff --git a/tests/test_schema.py b/tests/test_schema.py
index c7e1e7159..9971cf7d4 100644
--- a/tests/test_schema.py
+++ b/tests/test_schema.py
@@ -510,6 +510,7 @@ def test_leftover_pair_names_leftover_map_rank_column(schema_db) -> None:
         columns = dict(cur.fetchall())
     assert columns["leftover_map_rank"] == "YES"
 
+
 def test_corporate_hierarchy_recursive_query_returns_correct_shape(schema_db) -> None:
     """The real product requirement: 'Acme Group -> Acme Electronics Korea
     -> Acme Electronics Gwangju Plant' must be walkable with one query,