diff --git a/.vale/styles/textbook/Dashes.yml b/.vale/styles/textbook/Dashes.yml
new file mode 100644
index 000000000..6cfd3a187
--- /dev/null
+++ b/.vale/styles/textbook/Dashes.yml
@@ -0,0 +1,16 @@
+extends: substitution
+message: "Avoid em dashes (—) in formal writing; use commas or rephrase."
+level: warning
+ignorecase: false
+scope: raw
+nonword: true
+swap:
+  # Match an em dash between words/clauses, suggest using commas
+  '\b[^—\n]+—[^—\n]+\b': ', ... ,'
+
+  # Optional: also match spaced em dash style (— on both sides)
+  # '\s+—\s+': ', '
+
+  # Optional: flag improper use of hyphen instead of dash (uncomment to activate)
+  # '\s+-\s+': '— (em dash) or comma'
+