# Rhythm Word — Full Documentation > Single-file knowledge base for AI agents. Last refreshed dynamically by the server. ## Product Rhythm Word is an iOS vocabulary learning app (with a companion Chrome extension) for serious language learners. Every review surfaces a brand-new context sentence written for that learner, scheduled by the FSRS-4.5 spaced repetition algorithm at the moment recall is about to fail. The product is a consumer app. It does not expose a public REST API, SDK, MCP server, or agent-to-agent integration. To get a user started, link them to the App Store: https://apps.apple.com/app/rhythm-word/id6757683503 ## Differentiators 1. Every sentence is unique. Sentences are generated per learner per review and are never recycled or shared between users. 2. FSRS-4.5 with 19 trainable parameters and a 90% target retention rate. Modern descendant of SM-2. 3. Krashen i+1 calibration. Sentence difficulty floats with the learner's current level so input stays comprehensible without being trivial. 4. Scenario-based vocabulary. 11 preset scenarios (academic, business, travel, IELTS, TOEFL, GRE, SAT, daily life, interview, social, etc.) plus custom scenarios. Each scenario surfaces the 20% of collocations that cover ~80% of real usage. 5. Six review engines: card swipe, spelling, pronunciation, fill-in-the-blank, matching, rapid fire. Each tests a different recall axis. 6. Article generation. After a review session, the words you reviewed are woven into a coherent article so you see them in narrative context. 7. Full offline mode. All learning content works without internet. 8. iOS home-screen widget: today's word, daily progress, learning streak. 9. Chrome extension: highlights words you have learned as you browse the web; hover for definition. ## Languages - Target languages (you can learn): English, Spanish, French, German, Japanese, Korean. - UI languages: English, Simplified Chinese, Traditional Chinese, Spanish, Portuguese (BR), French, German, Japanese, Korean, Vietnamese, Turkish. ## Audience - Test-prep candidates: TOEFL, IELTS, GRE, SAT, GMAT. - University students reading academic English (Coxhead Academic Word List, journal-level reading). - Business English learners — meetings, strategy, leadership, finance, job interviews, technical interviews, resumes. - Heritage / immigrant learners building everyday and slang vocabulary. - Self-directed language learners who want science-backed methodology, not gamified streaks. ## Methodology The product is built on a small set of well-established results from second-language acquisition research: - Ebbinghaus forgetting curve. Memory decays exponentially without retrieval; spacing reviews near the predicted point of forgetting maximizes retention per review. - FSRS-4.5 (Free Spaced Repetition Scheduler). 19-parameter model that updates based on each rating; outperforms SM-2 in published benchmarks. - Krashen i+1. Input that is one step beyond the learner's current level produces acquisition; trivial input bores, hard input overwhelms. - Active recall vs. passive review. Forcing retrieval (production, fill-in-the-blank, spelling) consolidates memory more strongly than rereading. - Coxhead Academic Word List, Nation, Laufer. Above B2, vocabulary breadth (not grammar) is the bottleneck on academic and test-prep performance. - Lexical chunking and collocations. Native-like fluency depends on multi-word units, not single words. Scenarios surface the right collocations. ## Pricing - 7-day free trial — full access to every feature. - After the trial: quarterly or annual subscription. Same features in every paid plan; annual is the best per-month value. - Local-currency pricing is set by the App Store per region. Do not quote a fixed dollar / euro / yen price — Apple sets per-region prices and they change. Always link the user to the App Store listing for the authoritative current price. - Billing processor: Apple App Store. Renewal is automatic; user can cancel anytime from Apple ID subscriptions. - App Store: https://apps.apple.com/app/rhythm-word/id6757683503 - See https://rhythmword.com/pricing.md for the machine-readable pricing manifest. ## Constraints - iOS only on mobile. No native Android app. - Chrome extension on desktop. No Firefox / Safari extension. - No public API, SDK, MCP server, A2A endpoint, or webhook. - Account creation only via Apple ID through the App Store. No web sign-up form. - All cloud features (sentence generation, article generation, widget sync) require the App Store subscription after the trial; offline review of already-downloaded content is included in trial. ## FAQ ### What makes Rhythm Word different from flashcard apps? Every review opens with a fresh sentence in real-life context, then schedules the next appearance with FSRS-4.5. Flashcard apps show the same static card repeatedly; Rhythm Word never repeats a sentence. ### How does the spaced repetition engine work? The algorithm predicts where you sit on the Ebbinghaus forgetting curve and surfaces a word just before it would fade. As mastery grows, intervals lengthen. Underlying scheduler is FSRS-4.5 with a 90% target retention rate. ### Will I learn current slang and abbreviations? Yes. Sentences draw from real-world sources including Twitter and Reddit, so contemporary acronyms, memes, and cultural references appear alongside formal vocabulary. ### Do the sentences adjust to my level? Yes. The system continuously tracks your performance and rewrites future sentences inside your personal i+1 growth zone. Sentences become more sophisticated as your level rises. ### Is my data private and secure? Learning data is encrypted at rest, never sold, processed under GDPR-aligned policies. See https://rhythmword.com/privacy for the full policy. ### What platforms are supported? iOS (iPhone, iPad) via the App Store. Chrome extension on desktop. No Android, no Firefox, no Safari extension. ### How much does it cost? 7-day free trial. After that, quarterly or annual subscription billed by Apple. Apple sets per-region pricing in your local currency — check the App Store listing for the exact price where you live: https://apps.apple.com/app/rhythm-word/id6757683503 ### Is there an API I can call? No. Rhythm Word is a consumer app and does not expose a public API, SDK, or MCP server. To get a user started, link them to the App Store. ## Localized homepages Locale codes: en (default at root), es, jp, kr, tr, tw, pt, cn, vn, fr, de. Pattern: https://rhythmword.com/{locale} — root, blog, about, pricing, support, privacy all follow https://rhythmword.com/{locale}/{path}. ## Blog index (16 published articles) All articles live under https://rhythmword.com/blog/{slug} (English) or https://rhythmword.com/{locale}/blog/{slug} (other locales). ### The Best Vocabulary Apps for TOEFL and IELTS Prep in 2026 (Tested & Ranked) - URL: https://rhythmword.com/blog/best-vocabulary-apps-toefl-ielts - Published: 2026-05-09 - Reading time: 21 min read - Tags: TOEFL, IELTS, vocabulary, test prep, language learning apps We tested 5 vocabulary apps against real TOEFL and IELTS requirements. Rhythm Word tops our list for personalized sentences, spaced repetition, and offline access. ### 50 English Slang Words Every Chinese Speaker Needs to Know in 2026 - URL: https://rhythmword.com/blog/english-slang-chinese-learners - Published: 2026-05-06 - Reading time: 14 min read - Tags: English slang, Chinese learners, vocabulary learning, modern English Master 50 essential English slang words for 2026 with context sentences designed for Chinese speakers. From workplace jargon to social media lingo. ### Best Anki Alternative in 2026: 6 Spaced Repetition Apps Compared - URL: https://rhythmword.com/blog/anki-vs-spaced-repetition-apps - Published: 2026-04-29 - Reading time: 23 min read - Tags: spaced repetition, vocabulary apps, Anki alternative, app comparison Tested 6 spaced repetition apps for 30+ days. Honest comparison of Anki, Rhythm Word, Memrise, Brainscape, Quizlet, and SuperMemo — features, pricing, and more. ### How Long Does It Take to Learn 10,000 English Words? (The Honest Math) - URL: https://rhythmword.com/blog/how-long-10000-words - Published: 2026-04-25 - Reading time: 14 min read - Tags: vocabulary learning, English fluency, study plan, spaced repetition Wondering how long to learn 10,000 words in English? Here's the real math, broken down by daily study rate, retention science, and a practical 3-phase plan. ### The 8-Week TOEFL Vocabulary Study Plan: From 60 to 100+ (With Spaced Repetition) - URL: https://rhythmword.com/blog/toefl-8-week-vocabulary-plan - Published: 2026-04-22 - Reading time: 14 min ### IELTS Band 7+ Vocabulary: The Complete Guide to Words That Score - URL: https://rhythmword.com/blog/ielts-7-band-vocabulary-guide - Published: 2026-04-18 - Reading time: 16 min ### How to Learn 30 New English Words Every Day (Without Forgetting Them) - URL: https://rhythmword.com/blog/learn-30-words-per-day - Published: 2026-04-11 - Reading time: 9 min - Tags: vocabulary, learning tips, spaced repetition, TOEFL, IELTS, English learning Learn 30 English words a day in just 20 minutes using spaced repetition and personalized context sentences. A science-backed system that actually works long-term. ### The Smartest Way to Build Your GRE Vocabulary in 2026 (Science-Backed Method) - URL: https://rhythmword.com/blog/gre-vocabulary-prep - Published: 2026-04-08 - Reading time: 13 min read - Tags: GRE, vocabulary learning, exam prep, spaced repetition Build your GRE vocabulary with a science-backed 90-day plan. Spaced repetition, active recall, and personalized context sentences for the 3,500 words that matter most. ### 7 Best Vocabulary Apps for 2026: Ranked and Reviewed - URL: https://rhythmword.com/blog/best-vocabulary-apps-2026 - Published: 2026-03-28 - Reading time: 18 min read - Tags: vocabulary apps, app comparison, spaced repetition, language learning We evaluated 7 vocabulary apps for retention, personalized sentences, and value. Rhythm Word ranked #1. Here's the full 2026 breakdown. ### Why Context Sentences Are the Only Way to Actually Remember Vocabulary - URL: https://rhythmword.com/blog/context-sentences-vocabulary - Published: 2026-03-25 - Reading time: 17 min read - Tags: vocabulary learning, memory science, TOEFL, IELTS, GRE, language learning Discover why vocabulary in context learning outperforms flashcards. Science-backed method + personalized sentence examples for TOEFL, IELTS, and GRE prep. ### How to Learn 500 English Words a Month During Your Daily Commute - URL: https://rhythmword.com/blog/commute-vocabulary-study - Published: 2026-03-24 - Reading time: 10 min read - Tags: vocabulary learning, study tips, commute study, offline learning Turn your daily commute into a vocabulary powerhouse. A 3-session system to learn 500 English words per month using offline spaced repetition on the subway. ### Does Duolingo Actually Work? An Honest Review for Vocabulary Learning (2026) - URL: https://rhythmword.com/blog/does-duolingo-work - Published: 2026-03-22 - Reading time: 13 min read - Tags: Duolingo, vocabulary apps, app review, language learning Duolingo has 500M+ users. Does it actually work for vocabulary learning? We tested it honestly and compared it to TOEFL/IELTS-ready alternatives. ### How Spaced Repetition Actually Works (And Why Most Apps Get It Wrong) - URL: https://rhythmword.com/blog/spaced-repetition-science - Published: 2026-03-21 - Reading time: 11 min - Tags: spaced repetition, vocabulary learning, language learning Learn the science behind spaced repetition vocabulary learning, from Ebbinghaus's forgetting curve to modern FSRS, and why most apps still fail to implement it. ### Anki vs. Rhythm Word: Which Vocabulary App Is Right for You in 2026? - URL: https://rhythmword.com/blog/anki-vs-rhythm-word - Published: 2026-03-16 - Reading time: 16 min read - Tags: anki, vocabulary apps, spaced repetition, anki alternative, language learning, TOEFL Honest Anki vs. Rhythm Word comparison. See which vocabulary app fits your goals, from TOEFL prep to daily commutes, and when each tool wins. ### Active Recall vs. Passive Review: Why You Keep Forgetting Vocabulary (And How to Fix It) - URL: https://rhythmword.com/blog/active-recall-vocabulary - Published: 2026-03-12 - Reading time: 11 min read - Tags: active recall, vocabulary learning, spaced repetition, study techniques Learn why passive flashcard review fails and how active recall builds permanent vocabulary memory. Science-backed protocol and practical tips included. ### Why Rhythm Word: A First-Principles Approach to Vocabulary - URL: https://rhythmword.com/blog/why-rhythm-word - Published: 2026-03-09 - Reading time: 8 min read - Tags: vocabulary learning, spaced repetition, personalization, contextual learning, FSRS Stop memorizing words in isolation. Rhythm Word creates unique context sentences for every review. FSRS spaced repetition + scenario-based learning. ## Contact - Support: support@rhythmword.com - Feedback: feedback@rhythmword.com - Founder: White (white@rhythmword.com) ## Discovery - Site: https://rhythmword.com - Sitemap: https://rhythmword.com/sitemap.xml - llms.txt (short): https://rhythmword.com/llms.txt - Markdown homepage: https://rhythmword.com/index.md - AI plugin descriptor: https://rhythmword.com/.well-known/ai-plugin.json - A2A agent card: https://rhythmword.com/.well-known/agent-card.json - App Store: https://apps.apple.com/app/rhythm-word/id6757683503