diff --git a/README.ko.md b/README.ko.md
index a6bdb45..4c1e24f 100644
--- a/README.ko.md
+++ b/README.ko.md
@@ -346,6 +346,9 @@ const [amount, setAmount] = createSignal('') // 정식 값
 예정된 항목은 없습니다 — 폼에 필요한 기능이 빠져 있다면
 [이슈](https://github.com/devslab-kr/numkey/issues)로 알려주세요.
 
+검토했지만 로드맵에 올리지 않은 것들(필드 내 접두/접미 표시, 미정 상태인
+음수 한글 병기 표기): [docs/backlog.md](./docs/backlog.md).
+
 ## 기여하기
 
 이슈·PR 환영합니다 — 개발 환경 셋업과 규칙(문자열 우선 값 모델, IME 안전,
diff --git a/README.md b/README.md
index 0d658d4..4b3a7b5 100644
--- a/README.md
+++ b/README.md
@@ -350,6 +350,9 @@ const [amount, setAmount] = createSignal('') // canonical
 Nothing else is planned — [open an issue](https://github.com/devslab-kr/numkey/issues)
 if your form needs something this doesn't do.
 
+Ideas we considered and why they are not here (in-field prefix/suffix, the
+unspecified Korean reading of negatives): [docs/backlog.md](./docs/backlog.md).
+
 ## Contributing
 
 Issues and PRs welcome — see [CONTRIBUTING.md](./CONTRIBUTING.md) for the dev
diff --git a/docs/backlog.md b/docs/backlog.md
new file mode 100644
index 0000000..2ff74cd
--- /dev/null
+++ b/docs/backlog.md
@@ -0,0 +1,64 @@
+# Backlog — considered, not committed
+
+Ideas that came up for numkey, with the reasoning. The
+[Roadmap](../README.md#roadmap) holds only what shipped; this file holds
+everything else, so the same discussion doesn't get re-run and so
+contributors can see what's wanted before opening a PR.
+
+numkey를 두고 검토한 항목과 그 근거입니다. README 로드맵에는 출시된 것만
+두고, 나머지는 여기 적어 같은 논의를 반복하지 않도록 합니다.
+
+**Last reviewed: 2026-08-09.** Signal at that point: **0 issues** ever
+opened, ~12 npm downloads/week. Nothing below was requested by a user.
+/ 이슈 0건, 주간 다운로드 12. 아래는 전부 사용자 요청이 아닙니다.
+
+## In-field prefix / suffix — the one real feature gap
+
+Showing the unit inside the input (`₩ 1,234,567`, `1,234 원`, `12 %`).
+numkey has **no** prefix/suffix support at all, while AutoNumeric and
+react-number-format both do — it is the clearest gap against the
+alternatives, and Korean business forms ask for it.
+필드 안에 단위를 함께 표시하는 기능. 경쟁 라이브러리는 다 갖고 있고
+한국 업무 폼에서 흔한 요구인데 numkey에는 전혀 없습니다.
+
+What it would take:
+
+- The **read side already works** — `parse('₩ 1,234,567원')` → `'1234567'`,
+  since parse drops everything that isn't a digit, sign or decimal mark.
+- The cost is **display + caret math**. The caret is the most delicate part
+  of this library (`countSignificant` / `caretIndex` count *significant
+  characters*), and affixes are non-significant characters that must never
+  be deletable, selectable-through, or counted. Backspace at the boundary
+  and select-all-then-type both need deciding.
+- Harder than the lakh grouping work, which needed no caret changes at all.
+
+Not scheduled: demand-gated. One real request and it moves to the roadmap.
+
+## Korean reading of negative amounts — unspecified
+
+`toKorean('-9876543')` currently returns `"-987만 6,543"`. The reading is
+meant to be human-readable text, so leaving the sign as a bare glyph is
+half-and-half; Korean would more often be read "마이너스 987만 6,543".
+Banking UIs differ, so this is a **product decision, not a bug** — but it
+is currently neither decided nor documented, and there is no test pinning
+it. Whichever way it goes, write it down and cover it.
+음수 병기 표기가 미정입니다. 지금은 `-987만 6,543`. "마이너스"가 맞는지는
+제품 판단이며, 정하고 문서화 + 테스트로 고정해야 합니다.
+
+## Deliberately NOT doing
+
+- **Indian lakh grouping** — shipped in v0.6, no longer a backlog item.
+- **Sharing a core package with [kokey](https://github.com/devslab-kr/kokey)** —
+  the two are siblings in the "-key" family but deliberately share no
+  runtime code; only build infra and conventions were copied. Extract an
+  input-core only if a third input library creates real three-way
+  duplication. / 공용 런타임 패키지는 세 번째 입력 라이브러리가 생겨
+  실제 3중 중복이 날 때만.
+
+## The rule we're applying
+
+With zero issues and low download numbers, more features will not move
+anything — discovery is the bottleneck, not capability. Wait for a real
+request, then build. An idea sitting in this file is not a promise.
+이슈 0건·낮은 다운로드 상황에서 기능을 더 얹어도 지표는 안 움직입니다.
+실제 요청이 오면 그때 만듭니다. 여기 적힌 항목은 약속이 아닙니다.