diff --git a/src/cli.rs b/src/cli.rs
index 7a2799d..016d64e 100644
--- a/src/cli.rs
+++ b/src/cli.rs
@@ -16,6 +16,36 @@ pub struct Cli {
     command: Option<Command>,
 }
 
+/// The dice grammar, printed after `storied roll --help`'s usage line. This
+/// mirrors the grammar in `src/dice.rs`; keep the two in sync.
+const ROLL_AFTER_HELP: &str = "\
+Notation is one or more terms joined by + and -, like `3d6-1 + 4d100+5`.
+A term is a dice group (XdY) or a flat modifier.
+
+A group's count is optional (`d20` is `1d20`); `%` means 100 (`d%` is
+`d100`). Valid die sizes: 2, 3, 4, 6, 8, 10, 12, 20, 100.
+
+Suffixes attach to a group, in any order, case-insensitive. Rerolls and
+minimums settle before a keep/drop suffix picks which dice count.
+  khN, klN   keep the highest/lowest N; bare kh/kl keeps 1
+  dhN, dlN   drop the highest/lowest N
+  roN        reroll a die once, keeping the new value
+  ro<N       reroll once when the roll is less than N
+  ro>N       reroll once when the roll is greater than N
+  rN         reroll a die until the roll no longer equals N
+  r<N        reroll until the roll is no longer less than N
+  r>N        reroll until the roll is no longer greater than N
+  miN        raise each die's value to at least N
+
+Examples:
+  2d20kh     advantage
+  2d20kl     disadvantage
+  4d6kh3     ability score, keeping the best 3 of 4
+  4d6dl1     the same roll as 4d6kh3, said the other way
+  2d6ro<3    Great Weapon Fighting
+  d20ro1     Halfling Luck
+  8d6mi2     Elemental Adept";
+
 #[derive(Subcommand)]
 enum Command {
     /// Manage the SRD rules pipeline.
@@ -24,6 +54,7 @@ enum Command {
         command: SrdCommand,
     },
     /// Roll dice notation once and print the result.
+    #[command(after_help = ROLL_AFTER_HELP)]
     Roll {
         /// Dice notation to roll, like `d20` or `2d6+3`.
         notation: String,
diff --git a/tests/roll.rs b/tests/roll.rs
index 19839d7..8572680 100644
--- a/tests/roll.rs
+++ b/tests/roll.rs
@@ -14,6 +14,22 @@ fn rolling_valid_notation_echoes_it_and_prints_a_total() {
     assert!(stdout.contains("total:"));
 }
 
+#[test]
+fn help_documents_the_suffix_syntax() {
+    let output = Command::cargo_bin("storied")
+        .unwrap()
+        .args(["roll", "--help"])
+        .output()
+        .unwrap();
+
+    assert!(output.status.success());
+    let stdout = String::from_utf8(output.stdout).unwrap();
+    assert!(stdout.contains("kh"));
+    assert!(stdout.contains("dl"));
+    assert!(stdout.contains("ro<"));
+    assert!(stdout.contains("mi"));
+}
+
 #[test]
 fn rolling_unparseable_notation_prints_the_error_and_fails() {
     let output = Command::cargo_bin("storied")