diff --git a/crates/domain/src/lib.rs b/crates/domain/src/lib.rs
index 0877499..59755c1 100644
--- a/crates/domain/src/lib.rs
+++ b/crates/domain/src/lib.rs
@@ -2,8 +2,10 @@ pub mod note;
 pub mod chord;
 pub mod song;
 pub mod ports;
+pub mod transposer;
 
 pub use note::Note;
 pub use chord::Chord;
 pub use song::{ChordPosition, LyricLine, Section, SectionKind, SongMeta, Song};
 pub use ports::{FetchError, ParseError, TabFetcherPort, TabParserPort, TabSource};
+pub use transposer::{ChordTransposer, TransposeError};
diff --git a/crates/domain/src/transposer.rs b/crates/domain/src/transposer.rs
new file mode 100644
index 0000000..4f80c3b
--- /dev/null
+++ b/crates/domain/src/transposer.rs
@@ -0,0 +1,133 @@
+use thiserror::Error;
+use crate::{Chord, Note, Song, Section, LyricLine, ChordPosition};
+
+pub struct ChordTransposer;
+
+#[derive(Debug, Error)]
+pub enum TransposeError {
+    #[error("Song has no original_key set")]
+    MissingOriginalKey,
+    #[error("Unrecognized key: {0}")]
+    UnrecognizedKey(String),
+}
+
+impl ChordTransposer {
+    pub fn transpose_chord(&self, chord: &Chord, semitones: i8) -> Chord {
+        let new_semitone = (chord.root.semitone() as i16 + semitones as i16).rem_euclid(12) as u8;
+        Chord {
+            root: Note::from_semitone(new_semitone),
+            descriptor: chord.descriptor.clone(),
+        }
+    }
+
+    pub fn transpose_song(&self, song: &Song, semitones: i8) -> Song {
+        Song {
+            meta: song.meta.clone(),
+            sections: song.sections.iter().map(|s| self.transpose_section(s, semitones)).collect(),
+        }
+    }
+
+    pub fn transpose_to_key(&self, song: &Song, target_key: &str) -> Result<Song, TransposeError> {
+        let original = song.meta.original_key.as_deref()
+            .ok_or(TransposeError::MissingOriginalKey)?;
+        let from = Note::parse(Self::root_of(original))
+            .ok_or_else(|| TransposeError::UnrecognizedKey(original.to_string()))?;
+        let to = Note::parse(Self::root_of(target_key))
+            .ok_or_else(|| TransposeError::UnrecognizedKey(target_key.to_string()))?;
+        let semitones = (to.semitone() as i16 - from.semitone() as i16).rem_euclid(12) as i8;
+        Ok(self.transpose_song(song, semitones))
+    }
+
+    fn root_of(key: &str) -> &str {
+        if key.len() >= 2 && (key.as_bytes()[1] == b'#' || key.as_bytes()[1] == b'b') {
+            &key[..2]
+        } else {
+            &key[..1]
+        }
+    }
+
+    fn transpose_section(&self, section: &Section, semitones: i8) -> Section {
+        Section {
+            kind: section.kind.clone(),
+            label: section.label.clone(),
+            lines: section.lines.iter().map(|l| self.transpose_line(l, semitones)).collect(),
+        }
+    }
+
+    fn transpose_line(&self, line: &LyricLine, semitones: i8) -> LyricLine {
+        LyricLine {
+            text: line.text.clone(),
+            chords: line.chords.iter().map(|cp| ChordPosition {
+                offset: cp.offset,
+                chord: self.transpose_chord(&cp.chord, semitones),
+            }).collect(),
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::{Chord, Note};
+
+    fn chord(s: &str) -> Chord { Chord::parse(s).unwrap() }
+
+    #[test]
+    fn up_two_semitones() {
+        let t = ChordTransposer;
+        let result = t.transpose_chord(&chord("Em"), 2);
+        assert_eq!(result.name(true), "F#m");
+    }
+
+    #[test]
+    fn down_two_semitones() {
+        let t = ChordTransposer;
+        let result = t.transpose_chord(&chord("Em"), -2);
+        assert_eq!(result.name(false), "Dm");
+    }
+
+    #[test]
+    fn up_prefers_sharps() {
+        let t = ChordTransposer;
+        let result = t.transpose_chord(&chord("G"), 1);
+        assert_eq!(result.name(true), "G#");
+    }
+
+    #[test]
+    fn down_prefers_flats() {
+        let t = ChordTransposer;
+        let result = t.transpose_chord(&chord("G"), -1);
+        assert_eq!(result.name(false), "Gb");
+    }
+
+    #[test]
+    fn zero_unchanged() {
+        let t = ChordTransposer;
+        let result = t.transpose_chord(&chord("Am7"), 0);
+        assert_eq!(result.descriptor.as_deref(), Some("m7"));
+        assert_eq!(result.root, Note::A);
+    }
+
+    #[test]
+    fn wraps_octave() {
+        let t = ChordTransposer;
+        let result = t.transpose_chord(&chord("B"), 1);
+        assert_eq!(result.name(true), "C");
+    }
+
+    #[test]
+    fn transpose_to_key() {
+        let t = ChordTransposer;
+        let meta = crate::SongMeta {
+            title: "Test".into(),
+            artist: "Test".into(),
+            capo: None,
+            original_key: Some("G".into()),
+            tuning: None,
+            tempo: None,
+        };
+        let song = crate::Song { meta, sections: vec![] };
+        let result = t.transpose_to_key(&song, "A").unwrap();
+        assert_eq!(result.sections.len(), 0);
+    }
+}