# OtoTheory > OtoTheory is a music theory and chord progression app for guitar, bass, keyboard, and DAW users. Build chord progressions with real-time playback, 50+ presets, and 19 groove patterns (with dedicated 4/4 + 6/8 + 12/8 time-signature grooves on iOS). Available as a free web tool and iOS app (Free / Pro at ¥500/month or ¥5,000/year, 2 months free — pricing varies by region). Created by TH Quest, Japan. OtoTheory helps musicians create chord progressions without music theory knowledge. The app uses a "DJ-style composition" approach — edit chords while the music keeps playing, hear your changes instantly, and learn theory naturally along the way. Key features include OtoTheory AI (AI-powered chord suggestions), key & scale detection, MIDI/audio export, and fretboard visualization for guitar, bass, and keyboard. ## Primary Pages - [Home](https://www.ototheory.com/): Main landing page — what OtoTheory is, key features, Free vs Pro comparison, links to web tools - [About](https://www.ototheory.com/about): What OtoTheory is, key features, pricing (Free/Pro), philosophy, and creator info - [Chord Progression Builder](https://www.ototheory.com/chord-progression): Free online chord progression builder with real-time playback, 50+ presets, 19 groove patterns (iOS adds dedicated 6/8 + 12/8 time-signature grooves) - [Chord Finder](https://www.ototheory.com/find-chords): Find diatonic chords in any key and scale, visualize on guitar/bass/keyboard fretboards - [Get Chords (Chord Detection)](https://www.ototheory.com/get-chords): OtoTheory detects chord progressions, key, and song sections from your own audio files or a live recording of your playing — entirely on-device on iPhone with no upload. DRM-protected streaming tracks from Apple Music or Spotify cannot be analyzed. iOS only. Then analyze, edit, play with grooves, and (for recordings) export MIDI/PDF. - [Pricing](https://www.ototheory.com/pricing): Compare Free and Pro plans, 7-day free trial (cancel anytime), feature comparison - [FAQ](https://www.ototheory.com/faq): Frequently asked questions about OtoTheory - [Learn](https://www.ototheory.com/learn): Music theory learning hub — intervals, chords, scales, diatonic harmony, ear training, improvisation (individual articles at /learn/[topic]) - [Guides](https://www.ototheory.com/guides): Practical, task-based how-to guides — how to find the chords of a song, record your own playing to get chords, build a chord progression, and more. Each guide compares the general methods fairly, then shows the fastest way to do it with OtoTheory (individual guides at /guides/[topic]). - [How to find the chords of a song](https://www.ototheory.com/guides/how-to-find-chords-of-a-song): the ways to find a song's chords (by ear with theory, chord sites, app detection), and how to do it fastest with OtoTheory chord detection. - [Record yourself and get the chords](https://www.ototheory.com/guides/record-yourself-to-get-chords): record your own guitar or singing to get chords, key, degrees and function; send it to Create mode for theory-based chord suggestions. - [How to make a chord progression](https://www.ototheory.com/guides/how-to-make-a-chord-progression): a beginner's three ways in (50+ presets, the key's diatonic chords, in-app Ideas suggestions) and how to build one in OtoTheory's Create mode. - [How to find the key of a song](https://www.ototheory.com/guides/how-to-find-the-key-of-a-song): the ways to find a song's key (from the resolving/tonic chord, matching chords against a key's diatonic chords, app detection) and how to detect it automatically with OtoTheory chord detection. - [What chord comes next](https://www.ototheory.com/guides/what-chord-comes-next): three ways to choose the next chord (by a chord's role tonic/subdominant/dominant, by common moves like V–I and ii–V–I, or with app suggestions) and how to get role-labeled next-chord suggestions in OtoTheory's Create mode Ideas tab. - [Progressions Gallery](https://www.ototheory.com/progressions): Interactive gallery of famous chord progressions (Axis, Canon, 50s, Sensitive Female / Axis Rotation, Royal Road / 王道進行, 12-bar Blues, and more). Each page explains how the progression works, lists songs that use it, and links to the OtoTheory app preset. - [Glossary](https://www.ototheory.com/glossary): Music theory glossary with essential terms for chords, scales, modes, harmony, rhythm, composition ## Learn Music Theory — Individual Articles Eighteen-part guide to music theory fundamentals, from intervals to modes — covering chords, scales, diatonic harmony, ear training, progressions, cadences, chord types, substitution, borrowed chords, slash chords, melody/scale relationship, pentatonic, tension chords, voicing, and church modes (Dorian/Mixolydian/Lydian/Phrygian). Each article includes practical examples using OtoTheory features. 1. What Is Music Theory? https://www.ototheory.com/learn/theory-intro Introduction to music theory as a "recipe" for songwriting. Beatles analysis, OtoTheory feature overview. 2. What Are Intervals? https://www.ototheory.com/learn/intervals Understanding the distance between notes. Foundation for chords and scales. 3. What Are Chords? https://www.ototheory.com/learn/chords How notes stack to create harmony. Major, minor, and beyond. 4. What Is a Key? https://www.ototheory.com/learn/key-center The tonal center of a song. How keys organize melody and harmony. 5. What Are Scales? https://www.ototheory.com/learn/scales Finding the right notes for melodies and solos. 8 scale types in OtoTheory. 6. What Is Diatonic? https://www.ototheory.com/learn/diatonic The "chord team" for any key. Roman numeral analysis and chord functions. 7. The Science of Ear Training https://www.ototheory.com/learn/ear-training Training your ear to recognize intervals, chords, and progressions. 8. Learning Chord Progressions https://www.ototheory.com/learn/progressions How to give your chord sequence a sense of drama. 9. What Are Cadences? — The 4 Endings That Punctuate Your Progressions https://www.ototheory.com/learn/cadences The four cadence types (Authentic V→I, Half ?→V, Deceptive V→vi, Plagal IV→I) explained with verified examples from Twinkle Twinkle, The Beatles, Cyndi Lauper, Leonard Cohen, and Hozier. 10. Expanding Your Chord Palette https://www.ototheory.com/learn/chord-types 7ths, sus, and add chords — same progressions, richer sound. 11. Dress Up Your Progressions with Substitute Chords https://www.ototheory.com/learn/substitution Same-function chord swapping, tonic/subdominant/dominant substitution, shared notes, bass line variation, combining with 7th chords. 12. Non-Diatonic Chords — Calling in Reinforcements https://www.ototheory.com/learn/borrowed-chords ♭VII for rock power, IVm (subdominant minor) for emotional shadow, V/V (secondary dominant) for momentum; parallel minor borrowing, spice-not-mistake. 13. Slash Chords Explained — How One Bass Note Transforms Your Progressions https://www.ototheory.com/learn/slash-chords 3 essential patterns: descending bass walkdown for drama, ascending bass walkup for hope, pedal point for stability/floating; voice leading, progression-first technique; While My Guitar Gently Weeps/Lean On Me/Tomorrow Never Knows examples. 14. Melody & Scales: Your Melody's Safe Zone https://www.ototheory.com/learn/melody-and-scales Stay in the scale and you stay in tune. Land on a chord tone and you land with intention. Plaza-and-bench metaphor for melody writing, steps vs. leaps, passing tones, pentatonic shortcut. 15. The Pentatonic Scale: 5 Notes, Infinite Possibilities https://www.ototheory.com/learn/pentatonic Cut 2 notes from the 7-note scale and suddenly everything sounds better. Why pentatonic works for solos, major vs. minor pentatonic, blues notes, country/pop/rock applications, Stairway to Heaven and Comfortably Numb solos. 16. Tension Chords: Adding Color with 9ths, 11ths & 13ths https://www.ototheory.com/learn/tension-chords One extra note on top of a 7th chord makes everything sound sophisticated. 9th/11th/13th explained, jazz vs pop application, avoid notes, voicing tips with City Pop / R&B examples. 17. Chord Voicing Explained: Why the Same Chord Can Sound Completely Different https://www.ototheory.com/learn/voicing Rearrange the notes and a plain C major wears a whole new face. Inversions, drop voicings, open vs closed voicings, voice leading, guitar/piano comparison. 18. Modes Explained: One Scale, Seven Different Worlds https://www.ototheory.com/learn/modes Same seven white keys, different home note = seven different moods. Piano-experiment intro distinguishes scales from modes (Ionian/Dorian/Phrygian/Lydian/Mixolydian/Aeolian/Locrian). Characteristic notes, brightness order, modal vs tonal progressions (vamps vs V→I), modal interchange. Song examples: Miles Davis "So What", Sweet Home Alabama, Simpsons theme, Metallica. ## Japanese Pages - [ホーム](https://www.ototheory.com/ja/): トップページ（日本語版） - [OtoTheoryについて](https://www.ototheory.com/ja/about): OtoTheoryについて（日本語版） - [コード進行ビルダー](https://www.ototheory.com/ja/chord-progression): 無料コード進行作成ツール（日本語版） - [コードファインダー](https://www.ototheory.com/ja/find-chords): ダイアトニックコード検索ツール（日本語版） - [コード検出（Get Chords）](https://www.ototheory.com/ja/get-chords): 手持ちの音源ファイルやその場で録音した演奏から、コード進行・キー・セクションを iPhone 上だけで自動検出（完全オンデバイス・アップロードなし）。Apple Music や Spotify など DRM 配信曲は非対応。iOS 専用。検出後は理論分析・編集・グルーヴ再生・（録音は）MIDI/PDF 書き出し。 - [プラン比較](https://www.ototheory.com/ja/pricing): iOS版プラン比較（無料＆Pro）、7日間トライアル - [よくある質問](https://www.ototheory.com/ja/faq): よくある質問（日本語版） - [学ぶ](https://www.ototheory.com/ja/learn): 音楽理論学習ハブ（日本語版） - [コード進行ギャラリー](https://www.ototheory.com/ja/progressions): 有名コード進行の解説ギャラリー（Axis / カノン / 50年代 / Sensitive Female (Axis 回転形) / 王道進行 / 12小節ブルース 他）。各ページで仕組み・使用曲・OtoTheory プリセットへの導線を掲載 - [用語集](https://www.ototheory.com/ja/glossary): 音楽理論用語集（日本語版） - [学習記事](https://www.ototheory.com/ja/learn): 音楽理論ガイド — 度数、コード、スケール、ダイアトニック・ハーモニー、耳コピ、アドリブ ## 音楽理論ガイド — 個別記事 音楽理論の基礎を18本の記事で解説：度数・コード・スケール・ダイアトニック・耳コピ・進行・カデンツ（終止形）・コードの種類・代理コード・ノンダイアトニック・スラッシュコード・メロディとスケール・ペンタトニック・テンションコード・ボイシング・モード（チャーチモード = ドリアン/ミクソリディアン/リディアン/フリジアン）まで。各記事にはOtoTheoryの機能紹介を含む。 1. 音楽理論とは？ https://www.ototheory.com/ja/learn/theory-intro レシピとしての音楽理論。ビートルズ楽曲分析、OtoTheory機能紹介。 2. 度数とは？ https://www.ototheory.com/ja/learn/intervals 音の距離（度数）を理解する。コードとスケールの基礎。 3. コードとは？ https://www.ototheory.com/ja/learn/chords 音を重ねてハーモニーを作る。メジャー、マイナー、その先へ。 4. キーとは？ https://www.ototheory.com/ja/learn/key-center 曲の中心音（キー）。メロディとハーモニーを整理する仕組み。 5. スケールとは？ https://www.ototheory.com/ja/learn/scales メロディやソロに使える音を見つける。OtoTheoryの8スケール。 6. ダイアトニックとは？ https://www.ototheory.com/ja/learn/diatonic キーに合うコードのチーム。ローマ数字分析とコード機能。 7. 耳コピの科学 https://www.ototheory.com/ja/learn/ear-training 度数・コード・進行を耳で聴き分けるトレーニング。 8. コード進行を学ぼう https://www.ototheory.com/ja/learn/progressions コードの並びにドラマを持たせる方法。 9. カデンツとは？──終止形の4種類で進行に「句読点」をつける https://www.ototheory.com/ja/learn/cadences 完全終止 (V→I) / 半終止 (?→V) / 偽終止 (V→vi) / 変終止 (IV→I) の4種類を「文章の句読点」アナロジーで解説。きらきら星・The Beatles・back number・Leonard Cohen の楽曲例。 10. コードの種類を広げよう https://www.ototheory.com/ja/learn/chord-types 7th・sus・addで、同じ進行がもっと「らしく」なる。 11. 代理コードで進行を着替えさせよう https://www.ototheory.com/ja/learn/substitution 同機能コードの入れ替え、トニック/サブドミナント/ドミナント代理、共通音、ベースライン変化、7thコードとの組み合わせ。 12. ノンダイアトニックコード入門 https://www.ototheory.com/ja/learn/borrowed-chords ♭VIIのロック感、IVmサブドミナントマイナーの切なさ、V/Vセカンダリードミナントの推進力、同主調マイナーからの借用、スパイスとしての使い方。 13. オンコード入門──ベース音だけで進行が滑らかになる https://www.ototheory.com/ja/learn/slash-chords スラッシュコード/分数コードの3パターン: 下降ベースラインの切なさ、上昇ベースラインの希望感、ペダルポイントの浮遊感、ヴォイスリーディング、コード進行ありきのテクニック、While My Guitar Gently Weeps/Lean On Me/Tomorrow Never Knowsの楽曲例。 14. メロディとスケール──スケールはメロディの安全地帯 https://www.ototheory.com/ja/learn/melody-and-scales スケール=広場、コードトーン=ベンチ。広場の中を走り回るのは自由、コードトーンに着地すれば落ち着く。メロディ作りの基本構造、コードが変わるとベンチが移動する、ペンタトニックという「近道マップ」への橋渡し。 15. ペンタトニックスケール入門──5音で広がる無限の可能性 https://www.ototheory.com/ja/learn/pentatonic 7音から2音抜くだけで、アドリブもメロディも自由になる。なぜ5音が外しにくいのか、メジャー/マイナーペンタトニック、ブルーノート、〈Stairway to Heaven〉〈Comfortably Numb〉のソロ分析。 16. テンションコード入門──9th・11th・13thで「色気」を加える https://www.ototheory.com/ja/learn/tension-chords コードに「もう1色」足すだけで、響きが一気に大人っぽくなる。9th/11th/13thの違い、ジャズ/ポップスでの使い分け、アボイドノート、シティポップ/R&Bの実例（プラスティック・ラブ等）。 17. ボイシングとは？──同じコードでも「並べ方」で響きが変わる https://www.ototheory.com/ja/learn/voicing 音を積む順番を変えるだけで、同じCが別の顔を見せる。転回形、ドロップボイシング、オープン/クローズドボイシング、ヴォイスリーディング、ギター/ピアノの違い。 18. モード入門 — 同じ7音から7つの世界が生まれる https://www.ototheory.com/ja/learn/modes ピアノの白鍵 7 音をそのまま使っても、 「ド」 から始めるか「ラ」 から始めるか「レ」 から始めるかで全く違う響きになる ― これがモードの正体。 イオニアン/ドリアン/フリジアン/リディアン/ミクソリディアン/エオリアン/ロクリアンの 7 モード、 特徴音、 明るさの順序、 モード進行(vamp)、 モーダルインターチェンジ。 楽曲例: Miles Davis 「So What」、 Sweet Home Alabama、 ザ・シンプソンズ テーマ、 Metallica。 ## Legal & Support Pages - [Subscription Terms](https://www.ototheory.com/subscription): Subscription terms — 7-day free trial, ¥500/month or ¥5,000/year (2 months free) auto-renewal via App Store - [Privacy Policy](https://www.ototheory.com/privacy): Privacy policy - [Terms of Use](https://www.ototheory.com/terms): Terms of use - [Support](https://www.ototheory.com/support): Support and contact - [Company](https://www.ototheory.com/company): About TH Quest (developer) ## Japanese Legal & Support Pages - [プライバシーポリシー](https://www.ototheory.com/ja/privacy): 独自サーバーに個人データを保存しない設計 - [利用規約](https://www.ototheory.com/ja/terms): 利用規約 - [特定商取引法に基づく表記](https://www.ototheory.com/ja/tokushoho): 事業者情報・販売条件・解約方法 (TH Quest 個人事業主) - [サポート](https://www.ototheory.com/ja/support): サポート・お問い合わせ（support@ototheory.com） ## Key Information - OtoTheory is a chord progression builder with AI-powered suggestions - Built by TH Quest, a creative studio in Japan - Supports guitar, bass, and keyboard - On-device song analysis (iOS app, v3): import a song from your library or files and OtoTheory detects its chords, key, and sections on-device — no upload, no account. Includes chord shapes (guitar & keyboard) synced to playback and manual beat/tempo correction. (This is separate from the copy-and-paste "AI Import" chord progression feature on web and iOS.) - No ads and no cross-app/site tracking on web or iOS - 19 groove patterns across 5 categories (Energy, Vibe, Unplugged, Traditional, Synth). iOS adds dedicated 6/8 and 12/8 time-signature grooves. - MIDI/audio export in 3 modes (Groove, Groove+Theory, Chords Only) - Free on web, Pro features on iOS (¥500/month or ¥5,000/year, 2 months free — pricing varies by region) - iOS App Store: https://apps.apple.com/app/ototheory/id6755084287 ## Pricing - Free Plan: $0. All core features: AI chord import, 50+ presets, 19 groove patterns (with dedicated 4/4 + 6/8 + 12/8 time-signature grooves on iOS), all chords (tensions, slash chords), OtoTheory AI (all modes: Bridge, Substitute, Advanced), section management, fretboard visualization (guitar/bass/keyboard), key & scale detection, 3 local saves, text export. - Pro Plan: ¥500/month or ¥5,000/year (2 months free) in Japan; pricing varies by region, shown in local currency on the App Store. MIDI/audio export — 3 modes (unlimited), unlimited saves + iCloud sync, Pro-exclusive grooves (additional groove patterns available only to Pro subscribers). - 7-day free trial available via App Store. All Pro features are included during trial (unlimited MIDI/Audio/PDF exports, unlimited saves, iCloud sync, etc.). Cancel anytime during trial at no charge through the App Store. - Pricing page: https://ototheory.com/pricing (EN), https://ototheory.com/ja/pricing (JA)