diff --git a/app/app/lib/mock.ts b/app/app/lib/mock.ts
new file mode 100644
index 0000000..544bb04
--- /dev/null
+++ b/app/app/lib/mock.ts
@@ -0,0 +1,144 @@
+import type { Song, SongSummary } from "./types";
+
+const OCEAN: Song = {
+  meta: {
+    title: "A Drop In The Ocean",
+    artist: "Ron Pope",
+    capo: null,
+    original_key: "Em",
+    tuning: null,
+    tempo: null,
+  },
+  sections: [
+    {
+      kind: "chorus",
+      label: "Chorus",
+      lines: [
+        {
+          text: "A drop in the ocean,",
+          chords: [{ offset: 0, chord: "Em" }, { offset: 12, chord: "C" }],
+        },
+        {
+          text: "A change in the weather,",
+          chords: [{ offset: 2, chord: "G" }, { offset: 16, chord: "D" }],
+        },
+        {
+          text: "I was praying that you and me might end up together.",
+          chords: [
+            { offset: 6, chord: "Em" },
+            { offset: 17, chord: "C" },
+            { offset: 33, chord: "G" },
+            { offset: 44, chord: "D" },
+          ],
+        },
+      ],
+    },
+    {
+      kind: "verse",
+      label: "Verse",
+      lines: [
+        {
+          text: "I don't wanna waste the weekend,",
+          chords: [{ offset: 0, chord: "C" }, { offset: 15, chord: "G" }],
+        },
+        {
+          text: "If you don't love me, pretend",
+          chords: [{ offset: 3, chord: "D" }, { offset: 21, chord: "Em" }],
+        },
+        {
+          text: "A few more hours, then it's time to go.",
+          chords: [{ offset: 2, chord: "C" }, { offset: 18, chord: "G" }, { offset: 30, chord: "D" }],
+        },
+      ],
+    },
+    {
+      kind: "bridge",
+      label: "Bridge",
+      lines: [
+        {
+          text: "Still I can't let you be,",
+          chords: [{ offset: 0, chord: "Am" }, { offset: 11, chord: "G" }, { offset: 13, chord: "D" }],
+        },
+      ],
+    },
+  ],
+};
+
+const NAKED: Song = {
+  meta: {
+    title: "Naked",
+    artist: "James Arthur",
+    capo: null,
+    original_key: "G",
+    tuning: null,
+    tempo: null,
+  },
+  sections: [
+    {
+      kind: "chorus",
+      label: "Chorus",
+      lines: [
+        {
+          text: "I'm not going to wait until you're done",
+          chords: [{ offset: 0, chord: "G" }, { offset: 18, chord: "Bm" }],
+        },
+        {
+          text: "Pretending you don't need anyone",
+          chords: [{ offset: 16, chord: "Em" }, { offset: 27, chord: "C" }],
+        },
+        {
+          text: "I'm standing here naked",
+          chords: [{ offset: 19, chord: "G" }],
+        },
+      ],
+    },
+    {
+      kind: "verse",
+      label: "Verse",
+      lines: [
+        {
+          text: "I lay awake thinking of all I wasted",
+          chords: [{ offset: 0, chord: "G" }, { offset: 22, chord: "Bm" }],
+        },
+        {
+          text: "All of the time we had I took for granted",
+          chords: [{ offset: 11, chord: "Em" }, { offset: 32, chord: "C" }],
+        },
+      ],
+    },
+  ],
+};
+
+const SONGS_MAP: Record<string, Song> = {
+  "song-ocean": OCEAN,
+  "song-naked": NAKED,
+};
+
+function previewChords(song: Song): string[] {
+  const seen = new Set<string>();
+  const result: string[] = [];
+  for (const section of song.sections) {
+    for (const line of section.lines) {
+      for (const cp of line.chords) {
+        if (!seen.has(cp.chord)) {
+          seen.add(cp.chord);
+          result.push(cp.chord);
+        }
+      }
+    }
+    if (result.length >= 5) break;
+  }
+  return result.slice(0, 5);
+}
+
+export const MOCK_SONGS: SongSummary[] = Object.entries(SONGS_MAP).map(
+  ([id, song]) => ({
+    id,
+    meta: song.meta,
+    preview_chords: previewChords(song),
+  })
+);
+
+export function getMockSong(id: string): Song | null {
+  return SONGS_MAP[id] ?? null;
+}
diff --git a/app/app/lib/transpose.ts b/app/app/lib/transpose.ts
new file mode 100644
index 0000000..da96576
--- /dev/null
+++ b/app/app/lib/transpose.ts
@@ -0,0 +1,34 @@
+import type { Song } from "./types";
+
+const NOTES_SHARP = ["C", "C#", "D", "D#", "E", "F", "F#", "G", "G#", "A", "A#", "B"];
+const NOTES_FLAT  = ["C", "Db", "D", "Eb", "E", "F", "Gb", "G", "Ab", "A", "Bb", "B"];
+
+function transposeChord(chord: string, semitones: number): string {
+  const match = chord.match(/^([A-G][#b]?)(.*)/);
+  if (!match) return chord;
+  const [, root, descriptor] = match;
+  const idx = NOTES_SHARP.indexOf(root) !== -1
+    ? NOTES_SHARP.indexOf(root)
+    : NOTES_FLAT.indexOf(root);
+  if (idx === -1) return chord;
+  const newIdx = ((idx + semitones) % 12 + 12) % 12;
+  const notes = semitones >= 0 ? NOTES_SHARP : NOTES_FLAT;
+  return notes[newIdx] + descriptor;
+}
+
+export function transposeSong(song: Song, semitones: number): Song {
+  if (semitones === 0) return song;
+  return {
+    ...song,
+    sections: song.sections.map((section) => ({
+      ...section,
+      lines: section.lines.map((line) => ({
+        ...line,
+        chords: line.chords.map((cp) => ({
+          ...cp,
+          chord: transposeChord(cp.chord, semitones),
+        })),
+      })),
+    })),
+  };
+}
diff --git a/app/app/lib/types.ts b/app/app/lib/types.ts
new file mode 100644
index 0000000..eb79291
--- /dev/null
+++ b/app/app/lib/types.ts
@@ -0,0 +1,37 @@
+export interface ChordPosition {
+  offset: number;
+  chord: string;
+}
+
+export interface LyricLine {
+  text: string;
+  chords: ChordPosition[];
+}
+
+export interface Section {
+  kind: string;
+  label: string | null;
+  lines: LyricLine[];
+}
+
+export interface SongMeta {
+  title: string;
+  artist: string;
+  capo: number | null;
+  original_key: string | null;
+  tuning: string | null;
+  tempo: number | null;
+}
+
+export interface Song {
+  meta: SongMeta;
+  sections: Section[];
+}
+
+// Trimmed version used in the library grid
+export interface SongSummary {
+  id: string;
+  meta: SongMeta;
+  // First 5 unique chord names from the song, in order of appearance
+  preview_chords: string[];
+}