diff --git a/README.md b/README.md
index d2c27b3..785bd59 100644
--- a/README.md
+++ b/README.md
@@ -1,24 +1,114 @@
 # Fontist Website
 
-[![Deploy Pages](https://github.com/fontist/fontist.github.io/actions/workflows/deploy-pages.yml/badge.svg)](https://github.com/fontist/fontist.github.io/actions/workflows/deploy-pages.yml)
+[![build](https://github.com/fontist/fontist.github.io/actions/workflows/build.yml/badge.svg)](https://github.com/fontist/fontist.github.io/actions/workflows/build.yml)
 
-This is the source for [fontist.org](https://www.fontist.org), built with [VitePress](https://vitepress.dev/).
+This is the source for [fontist.org](https://www.fontist.org), built with [VitePress](https://vitepress.dev/). It is the **root** of a family of sites that together cover the Fontist project — this repo hosts the landing page, blog, and about pages, while each tool's documentation lives in its own repository and is deployed to a subpath of `fontist.org`.
+
+## Site architecture
+
+`fontist.org` is a **multi-repo, multi-site** setup. This repo (`fontist/fontist.github.io`) is the GitHub Pages **org site** that owns the `fontist.org` custom domain and is served at the apex (`/`). Each documentation subsite lives in its **own repository**, deploys its **own** GitHub Pages, and is served at `fontist.org/<repo>/` via GitHub Pages' project-page-under-custom-domain routing.
+
+```mermaid
+flowchart TD
+    subgraph repos["Repositories — each builds & deploys independently"]
+        direction LR
+        R0["<b>fontist/fontist.github.io</b><br/>(this repo)<br/>VitePress · build.yml"]
+        R1["fontist/fontist<br/>(docs/)<br/>VitePress · docs.yml"]
+        R2["fontist/fontisan<br/>(docs/)<br/>VitePress · docs.yml"]
+        R3["fontist/formulas<br/>(docs/)<br/>VitePress + custom build · docs.yml"]
+    end
+    subgraph site["fontist.org (custom domain)"]
+        direction LR
+        U0["/<br/>landing · blog · about"]
+        U1["/fontist/<br/>Fontist gem docs"]
+        U2["/fontisan/<br/>Fontisan gem docs"]
+        U3["/formulas/<br/>formula index (4,300+)"]
+    end
+    R0 -->|GitHub Pages| U0
+    R1 -->|GitHub Pages| U1
+    R2 -->|GitHub Pages| U2
+    R3 -->|GitHub Pages| U3
+    U0 -.->|nav links| U1
+    U0 -.->|nav links| U2
+    U0 -.->|nav links| U3
+```
+
+**How the routing works:** `fontist.github.io` owns the custom domain. Each subsite repo enables GitHub Pages with the same custom domain, so GitHub serves it at `fontist.org/<repo>/` — as long as this root repo does **not** publish content at those paths. Each subsite sets `base: "/<subsite>/"` in its VitePress config so generated asset/link paths include the subpath.
+
+| Site | URL | Source repo | Default branch | Build entrypoint |
+|---|---|---|---|---|
+| Root (landing, blog, about) | `fontist.org/` | `fontist/fontist.github.io` *(this repo)* | `main` | `npm run build` |
+| Fontist docs | `fontist.org/fontist/` | [`fontist/fontist`](https://github.com/fontist/fontist) (`docs/`) | `main` | `npm run build` *(in `docs/`)* |
+| Fontisan docs | `fontist.org/fontisan/` | [`fontist/fontisan`](https://github.com/fontist/fontisan) (`docs/`) | `main` | `npm run build` *(in `docs/`)* |
+| Formulas index | `fontist.org/formulas/` | [`fontist/formulas`](https://github.com/fontist/formulas) (`docs/`) | `v5` | `npm run build` *(in `docs/`, custom `build.js`)* |
+
+> The Fontist GitHub Action ([`fontist/setup-fontist`](https://github.com/fontist/setup-fontist)) is a separate repo for CI/CD font installation and is not part of the docs site graph.
 
 ## Development
 
+### This site (root)
+
+```bash
+npm install
+npm run dev       # start the local dev server (prints the local URL)
+npm run build     # builds to .vitepress/dist (includes post-build "dirify" step)
+npm run preview   # preview the production build
+```
+
+### Subsites
+
+The subsites live in sibling repositories. Clone them alongside this repo:
+
 ```bash
-npm install     # Install dependencies
-npm run dev     # Start development server
-npm run build   # Build for production
-npm run preview # Preview production build
+git clone https://github.com/fontist/fontist.git  ../fontist
+git clone https://github.com/fontist/fontisan.git ../fontisan
+git clone https://github.com/fontist/formulas.git ../formulas
 ```
 
-## Adding Blog Posts
+Then, from the subsite's `docs/` directory:
+
+```bash
+cd ../fontist/docs     # or ../fontisan/docs, ../formulas/docs
+npm install
+npm run dev            # local dev server
+npm run build          # builds to docs/.vitepress/dist (+ post-build steps)
+```
+
+The `base` path defaults to the subsite's subpath (`/fontist/`, `/fontisan/`, `/formulas/`) via `process.env.BASE_PATH` in each subsite's `.vitepress/config.ts`, so a plain `npm run dev` works out of the box.
+
+## Deployment
+
+Every site deploys **automatically** when changes are pushed/merged to its default branch (`main`, or `v5` for formulas). There is nothing to deploy manually — open a PR in the relevant repo, merge it, and GitHub Pages handles the rest.
+
+| Repo | Workflow | What it does |
+|---|---|---|
+| `fontist/fontist.github.io` | [`build.yml`](.github/workflows/build.yml) | build → verify → upload artifact → deploy to Pages |
+| `fontist/fontist` | `docs.yml` | same pattern (runs in `docs/`) |
+| `fontist/fontisan` | `docs.yml` | same pattern (runs in `docs/`) |
+| `fontist/formulas` | `docs.yml` | batched build (4,300+ pages) → combine → deploy |
+
+Each site builds to static HTML and deploys via `actions/upload-pages-artifact` + `actions/deploy-pages`.
+
+## Adding blog posts
+
+1. Create a new `.md` file in [`blog/`](blog/) with frontmatter (`title`, `description`, `authors`, optional `date`).
+2. Add an entry to [`blog/index.md`](blog/index.md).
+
+Blog posts get `BlogPosting` JSON-LD structured data automatically via the `transformHead` hook in [.vitepress/config.ts](.vitepress/config.ts).
+
+## Conventions shared across all sites
+
+For consistency, every site in the ecosystem follows these conventions. If you add a new site or change one, keep these aligned:
 
-1. Create a new `.md` file in `blog/` with frontmatter
-2. Add an entry to `blog/index.md`
+- **Per-page Open Graph / Twitter tags** — each site derives `og:title`, `og:description`, `og:url` from the page's frontmatter/H1 via a `transformHead` hook in `.vitepress/config.ts`, rather than using a single site-wide value.
+- **Shared social card** — `og:image` points to `https://www.fontist.org/og-image.png` (PNG; SVG has poor support on Twitter/Facebook/LinkedIn).
+- **`sitemap.xml` + `robots.txt`** — each site generates both. Subsites post-process the sitemap to insert their `base` path, because VitePress omits `base` from sitemap routes.
+- **Directory-style URLs** — a post-build step (`scripts/post-build.mjs` here and in fontist/fontisan; custom `build.js` in formulas) converts `foo.html` → `foo/index.html` so both `/foo` and `/foo/` resolve on GitHub Pages (which otherwise 404s on the trailing slash for file routes).
+- **Cross-site links carry `target="_self"`** (in `nav`/`sidebar` config) so VitePress's client-side router doesn't intercept same-origin clicks and render a client-side 404. Inline markdown links to other sites in the domain are auto-tagged `target="_blank"` by VitePress.
 
-## Sub-sites
+## See also
 
-- [fontist.org/fontist/](https://www.fontist.org/fontist/) - Main documentation (from [fontist/fontist](https://github.com/fontist/fontist))
-- [fontist.org/formulas/](https://www.fontist.org/formulas/) - Searchable formulas index (from [fontist/formulas](https://github.com/fontist/formulas))
+- [fontist/fontist](https://github.com/fontist/fontist) — the Fontist gem and its documentation.
+- [fontist/fontisan](https://github.com/fontist/fontisan) — the Fontisan gem and its documentation.
+- [fontist/formulas](https://github.com/fontist/formulas) — the formula registry and searchable index.
+- [fontist/setup-fontist](https://github.com/fontist/setup-fontist) — the official GitHub Action for installing Fontist in CI.