diff --git a/.github/scripts/validate_data.py b/.github/scripts/validate_data.py
new file mode 100644
index 0000000..749ec85
--- /dev/null
+++ b/.github/scripts/validate_data.py
@@ -0,0 +1,357 @@
+#!/usr/bin/env python3
+"""Validate the OSHB data files and summarise what a change touches.
+
+Usage: validate_data.py [--root DIR] [--base DIR]
+
+Checks (any failure exits 1):
+  * every tracked .xml file is well-formed
+  * wlc/: 39 books, the expected verse inventory, every <w> is inside a
+    verse and carries a lemma, a morph code and a unique five-character id
+  * MAPM/MAPM.xml: present, verse milestones are balanced, and its verse
+    inventory matches wlc/ (apart from the verses MAM's text does not have)
+  * no operating-system junk files are tracked
+
+With --base (a checkout of the revision being merged into) it also writes a
+summary of which words and verses changed, to stdout and, on GitHub Actions,
+to the job summary. The summary never fails the build: problems in the base
+revision, or in producing the summary, are reported in the summary only.
+
+Standard library only.
+"""
+import argparse
+import collections
+import os
+import re
+import subprocess
+import sys
+import unicodedata
+import xml.etree.ElementTree as ET
+
+NS = "{http://www.bibletechnologies.net/2003/OSIS/namespace}"
+WLC_BOOKS = 39
+WLC_VERSES = 23213
+# Verses that BHS numbers but MAM's text does not have.
+MAPM_ABSENT = {"Josh.21.36", "Josh.21.37"}
+JUNK = re.compile(r"(^|/)(Thumbs\.db|\.DS_Store|desktop\.ini)$", re.I)
+# These are applied with fullmatch(): "$" alone would accept a trailing newline.
+MORPH = re.compile(r"[HA][A-Za-z0-9]+(/[A-Za-z0-9]+)*")
+STRUCT = re.compile(r"\d+(\.\d+)*")
+WORD_ID = re.compile(r"[0-9]{2}[A-Za-z0-9]{3}")
+VERSE_ID = re.compile(r"[1-2]?[A-Za-z]+\.\d+\.\d+")
+WORD_FIELDS = ("text", "lemma", "morph", "n", "type", "verse")
+MAX_ROWS = 200
+
+errors = []
+parsed = set()  # absolute paths already checked for well-formedness
+
+
+def error(msg):
+    errors.append(msg)
+
+
+def tracked_files(root):
+    try:
+        out = subprocess.run(["git", "-C", root, "ls-files", "-z"], check=True,
+                             capture_output=True).stdout.decode("utf-8")
+        return [f for f in out.split("\0") if f]
+    except (OSError, subprocess.CalledProcessError):
+        found = []
+        for base, dirs, files in os.walk(root):
+            dirs[:] = [d for d in dirs if d != ".git"]
+            found += [os.path.relpath(os.path.join(base, f), root) for f in files]
+        return found
+
+
+def parse(path, label=None, quiet=False):
+    """Parse an XML file. Returns its root, or None if it cannot be parsed.
+
+    A failure is a validation error unless quiet is set, which is how the
+    base revision is read: its problems must not fail the current one.
+    """
+    if not quiet:
+        parsed.add(os.path.abspath(path))
+    try:
+        return ET.parse(path).getroot()
+    except ET.ParseError as exc:
+        if not quiet:
+            error(f"{label or path}: not well-formed XML: {exc}")
+    except OSError as exc:
+        if not quiet:
+            error(f"{label or path}: cannot read: {exc}")
+    return None
+
+
+def tag(el):
+    return el.tag.replace(NS, "")
+
+
+# ---------------------------------------------------------------- wlc/
+
+Wlc = collections.namedtuple("Wlc", "words verses order unreadable")
+
+
+def load_wlc(root, check):
+    """Read the wlc/ books under root.
+
+    words: {word id: record}; verses: [verse ids]; order: {verse id: [word
+    ids]}; unreadable: book files that could not be parsed. With check set,
+    problems are recorded as validation errors.
+    """
+    words, verses, order, unreadable = {}, [], {}, []
+    wlc = os.path.join(root, "wlc")
+    books = sorted(f for f in os.listdir(wlc)
+                   if f.endswith(".xml") and f != "VerseMap.xml") if os.path.isdir(wlc) else []
+    if check and len(books) != WLC_BOOKS:
+        error(f"wlc/: expected {WLC_BOOKS} book files, found {len(books)}")
+    for name in books:
+        rel = f"wlc/{name}"
+        doc = parse(os.path.join(wlc, name), label=rel, quiet=not check)
+        if doc is None:
+            unreadable.append(rel)
+            continue
+        book = name[:-4]
+        in_verse = set()
+        for verse in doc.iter(NS + "verse"):
+            vid = verse.get("osisID", "")
+            verses.append(vid)
+            ids = order.setdefault(vid, [])
+            if check and (not VERSE_ID.fullmatch(vid) or vid.split(".")[0] != book):
+                error(f"{rel}: unexpected verse osisID {vid!r}")
+            for w in verse.iter(NS + "w"):
+                in_verse.add(w)
+                wid = w.get("id")
+                rec = {"verse": vid, "book": rel, "text": "".join(w.itertext()),
+                       "lemma": w.get("lemma"), "morph": w.get("morph"),
+                       "n": w.get("n"), "type": w.get("type")}
+                if check:
+                    where = f"{rel} {vid} word {wid!r}"
+                    if wid is None or not WORD_ID.fullmatch(wid):
+                        error(f"{where}: missing or malformed id")
+                    elif wid in words:
+                        error(f"{where}: duplicate id (also in {words[wid]['verse']})")
+                    if not rec["lemma"]:
+                        error(f"{where}: missing lemma")
+                    if not rec["morph"] or not MORPH.fullmatch(rec["morph"]):
+                        error(f"{where}: missing or malformed morph {rec['morph']!r}")
+                    if rec["n"] is not None and not STRUCT.fullmatch(rec["n"]):
+                        error(f"{where}: malformed n {rec['n']!r}")
+                    if not rec["text"].strip():
+                        error(f"{where}: empty word")
+                if wid is not None:
+                    ids.append(wid)
+                    words.setdefault(wid, rec)
+        if check:
+            for w in doc.iter(NS + "w"):
+                if w not in in_verse:
+                    text = "".join(w.itertext()).strip()
+                    error(f"{rel}: word outside any verse (id {w.get('id')!r}, text {text[:20]!r})")
+    if check:
+        if len(verses) != WLC_VERSES:
+            error(f"wlc/: expected {WLC_VERSES} verses, found {len(verses)}")
+        if len(set(verses)) != len(verses):
+            dup = [v for v, c in collections.Counter(verses).items() if c > 1]
+            error(f"wlc/: duplicate verse ids: {dup[:5]}")
+    return Wlc(words, verses, order, unreadable)
+
+
+# ---------------------------------------------------------------- MAPM/
+
+def load_mapm(root, check):
+    """Return {verse id: text} for MAPM/MAPM.xml, or None if absent or unparseable.
+
+    With check set, problems (including the file being absent) are recorded
+    as validation errors.
+    """
+    rel = "MAPM/MAPM.xml"
+    path = os.path.join(root, "MAPM", "MAPM.xml")
+    if not os.path.exists(path):
+        if check:
+            error(f"{rel}: file is missing")
+        return None
+    doc = parse(path, label=rel, quiet=not check)
+    if doc is None:
+        return None
+    verses, current, skip = collections.OrderedDict(), None, 0
+
+    def walk(el):
+        nonlocal current, skip
+        name = tag(el)
+        if name == "verse":
+            start, end = el.get("sID"), el.get("eID")
+            if check and bool(start) == bool(end):
+                which = "both sID and eID" if start else "neither sID nor eID"
+                error(f"{rel}: verse milestone {start or el.get('osisID')!r} has {which}")
+            if start:
+                if check and current is not None:
+                    error(f"{rel}: verse {start} starts before {current} ends")
+                current = start
+                if check and current in verses:
+                    error(f"{rel}: duplicate verse {current}")
+                verses[current] = []
+            elif end:
+                if check and end != current:
+                    error(f"{rel}: verse end {end} does not match open verse {current}")
+                current = None
+        muted = name in ("note", "title", "header")
+        skip += muted
+        if el.text and not skip:
+            text(el.text)
+        for child in el:
+            walk(child)
+            if child.tail and not skip:
+                text(child.tail)
+        skip -= muted
+
+    def text(s):
+        if current is not None:
+            verses[current].append(s)
+        elif check and s.strip():
+            error(f"{rel}: text outside any verse: {s.strip()[:30]!r}")
+
+    walk(doc)
+    if check and current is not None:
+        error(f"{rel}: verse {current} is never closed")
+    return {v: re.sub(r"\s+", " ", "".join(parts)).strip() for v, parts in verses.items()}
+
+
+def letters(s):
+    return "".join(c for c in s if "א" <= c <= "ת")
+
+
+# ---------------------------------------------------------------- report
+
+def clip(items, limit):
+    return ", ".join(items[:limit]) + (" …" if len(items) > limit else "")
+
+
+def report(base, wlc, mapm):
+    """Markdown summary of what differs from the base revision.
+
+    The base is read quietly: nothing here adds to the validation errors.
+    """
+    out = ["## Data change summary", "", "### wlc/"]
+    old = load_wlc(base, check=False)
+    skipped = set(old.unreadable)
+    if skipped:
+        out.append(f"- not compared, because the base revision's copy could not be read: "
+                   f"{clip(sorted(skipped), 40)}")
+    words = {i: r for i, r in wlc.words.items() if r["book"] not in skipped}
+    added = [i for i in words if i not in old.words]
+    removed = [i for i in old.words if i not in wlc.words]
+    changed = []
+    for wid, rec in words.items():
+        before = old.words.get(wid)
+        if before is None:
+            continue
+        fields = [f for f in WORD_FIELDS if rec[f] != before[f]]
+        if fields:
+            changed.append((wid, rec, before, fields))
+    reordered = [v for v, ids in wlc.order.items()
+                 if v in old.order and ids != old.order[v] and sorted(ids) == sorted(old.order[v])]
+    if not (added or removed or changed or reordered):
+        out.append("No word-level changes." if not skipped else "No word-level changes in the books compared.")
+    else:
+        per_field = collections.Counter(f for _, _, _, fs in changed for f in fs)
+        out.append(f"- words changed: **{len(changed)}** "
+                   f"({', '.join(f'{f}: {n}' for f, n in per_field.most_common()) or 'none'})")
+        out.append(f"- word ids added: **{len(added)}**, removed: **{len(removed)}**")
+        out.append(f"- verses whose words are the same but in a different order: **{len(reordered)}**"
+                   + (f" — {clip(reordered, 60)}" if reordered else ""))
+        if changed:
+            out += ["", "| verse | id | field | before | after |", "|---|---|---|---|---|"]
+            rows = [(rec["verse"], wid, f, before[f], rec[f])
+                    for wid, rec, before, fs in changed for f in fs]
+            for v, wid, f, a, b in rows[:MAX_ROWS]:
+                out.append(f"| {v} | `{wid}` | {f} | {a if a is not None else '(none)'} "
+                           f"| {b if b is not None else '(none)'} |")
+            if len(rows) > MAX_ROWS:
+                out.append(f"| … | | | {len(rows) - MAX_ROWS} more rows not shown | |")
+            out.append("")
+        for label, ids, src in (("added", added, words), ("removed", removed, old.words)):
+            if ids:
+                shown = clip(["`%s` (%s)" % (i, src[i]["verse"]) for i in ids], 30)
+                out.append(f"- {label}: {shown}")
+    out += ["", "### MAPM/"]
+    old_mapm = load_mapm(base, check=False)
+    if mapm is None or old_mapm is None:
+        out.append("MAPM/MAPM.xml is missing or unreadable on one side; not compared.")
+    elif mapm == old_mapm:
+        out.append("No verse text changes.")
+    else:
+        shared = [v for v in mapm if v in old_mapm]
+        differ = [v for v in shared if mapm[v] != old_mapm[v]]
+        marks_only = [v for v in differ
+                      if unicodedata.normalize("NFD", mapm[v]).replace(" ", "")
+                      == unicodedata.normalize("NFD", old_mapm[v]).replace(" ", "")]
+        cons = [v for v in differ if letters(mapm[v]) != letters(old_mapm[v])]
+        out.append(f"- verses only in this revision: **{len(set(mapm) - set(old_mapm))}**, "
+                   f"only in base: **{len(set(old_mapm) - set(mapm))}**")
+        out.append(f"- shared verses whose text differs: **{len(differ)}** of {len(shared)} "
+                   f"({len(marks_only)} differ only in spacing or mark order)")
+        out.append(f"- of those, verses whose consonants differ: **{len(cons)}**"
+                   + (f" — {clip(cons, 60)}" if cons else ""))
+    return "\n".join(out) + "\n"
+
+
+# ---------------------------------------------------------------- main
+
+def main():
+    ap = argparse.ArgumentParser(description=__doc__.split("\n")[0])
+    ap.add_argument("--root", default=".")
+    ap.add_argument("--base", help="checkout of the base revision, for the change summary")
+    args = ap.parse_args()
+    root = args.root
+
+    files = tracked_files(root)
+    for f in files:
+        if JUNK.search(f):
+            error(f"{f}: operating-system junk file is tracked; please remove it")
+
+    wlc = load_wlc(root, check=True)
+    mapm = load_mapm(root, check=True)
+    if mapm is not None and wlc.verses:
+        expected = set(wlc.verses) - MAPM_ABSENT
+        got = set(mapm)
+        if got != expected:
+            extra, missing = sorted(got - expected), sorted(expected - got)
+            error(f"MAPM/MAPM.xml: verse inventory differs from wlc/ "
+                  f"(unexpected: {extra[:8]}{' …' if len(extra) > 8 else ''}; "
+                  f"missing: {missing[:8]}{' …' if len(missing) > 8 else ''})")
+
+    # Every other tracked XML file, whatever it is, must at least be well-formed.
+    for f in files:
+        path = os.path.join(root, f)
+        if f.lower().endswith(".xml") and os.path.abspath(path) not in parsed:
+            if os.path.exists(path):
+                parse(path, label=f)
+
+    print(f"wlc/: {len(wlc.verses)} verses, {len(wlc.words)} words")
+    if mapm is not None:
+        print(f"MAPM/: {len(mapm)} verses")
+
+    if args.base:
+        try:
+            summary = report(args.base, wlc, mapm)
+        except Exception as exc:  # the summary is a convenience and must never fail the run
+            summary = f"## Data change summary\n\nThe summary could not be produced: {exc!r}\n"
+        print("\n" + summary)
+        target = os.environ.get("GITHUB_STEP_SUMMARY")
+        if target:
+            with open(target, "a", encoding="utf-8") as fh:
+                fh.write(summary)
+
+    if errors:
+        print(f"\nFAILED: {len(errors)} problem(s)")
+        for e in errors[:100]:
+            print(f"  - {e}")
+            if os.environ.get("GITHUB_ACTIONS"):
+                print(f"::error::{e}")
+        if len(errors) > 100:
+            print(f"  … and {len(errors) - 100} more")
+        return 1
+    print("\nOK: all checks passed")
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())
diff --git a/.github/workflows/validate.yml b/.github/workflows/validate.yml
new file mode 100644
index 0000000..ae56234
--- /dev/null
+++ b/.github/workflows/validate.yml
@@ -0,0 +1,31 @@
+name: Validate data
+
+on:
+  pull_request:
+  push:
+    branches: [master]
+
+permissions:
+  contents: read
+
+jobs:
+  validate:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - name: Check out the base revision for comparison
+        if: github.event_name == 'pull_request'
+        env:
+          BASE_SHA: ${{ github.event.pull_request.base.sha }}
+        run: git worktree add --detach ../base "$BASE_SHA"
+
+      - name: Validate (pull request)
+        if: github.event_name == 'pull_request'
+        run: python3 .github/scripts/validate_data.py --base ../base
+
+      - name: Validate (push)
+        if: github.event_name != 'pull_request'
+        run: python3 .github/scripts/validate_data.py
diff --git a/.gitignore b/.gitignore
index 32ba899..2d6383d 100644
--- a/.gitignore
+++ b/.gitignore
@@ -20,3 +20,8 @@
 json/
 remapped.json
 hebrew.json
+
+# Operating-system junk
+Thumbs.db
+.DS_Store
+desktop.ini
diff --git a/HomeFiles/Thumbs.db b/HomeFiles/Thumbs.db
deleted file mode 100644
index 8194828..0000000
Binary files a/HomeFiles/Thumbs.db and /dev/null differ
diff --git a/OSHB Graphics/Thumbs.db b/OSHB Graphics/Thumbs.db
deleted file mode 100644
index 677fdb2..0000000
Binary files a/OSHB Graphics/Thumbs.db and /dev/null differ
diff --git a/read/Image/Thumbs.db b/read/Image/Thumbs.db
deleted file mode 100644
index 4b6fc3c..0000000
Binary files a/read/Image/Thumbs.db and /dev/null differ
diff --git a/structure/OshbVerse/Image/Thumbs.db b/structure/OshbVerse/Image/Thumbs.db
deleted file mode 100644
index acadec0..0000000
Binary files a/structure/OshbVerse/Image/Thumbs.db and /dev/null differ