Quick answers to common questions.
Underlayer is a learning platform designed for deep understanding of technical subjects. Unlike typical course platforms, every concept is verified against authoritative sources, taught from first principles, and reinforced through spaced repetition.
Yes. Underlayer is open source and free to use. Create an account to save your progress across devices.
Underlayer uses FSRS (Free Spaced Repetition Scheduler), an algorithm that calculates the optimal time to review each concept based on your past performance. It adapts to your individual memory patterns — concepts you find easy are shown less often, while difficult ones appear more frequently.
Knowledge Health is a score (0-100) that estimates how well you've retained all concepts you've studied. It factors in review accuracy, time since last review, and difficulty ratings. A score above 80 means your knowledge is stable.
Yes. Course content is compiled to static HTML/CSS/JS files. You can download them and study without an internet connection. Progress will be saved to localStorage and synced when you're back online.
Underlayer prioritizes depth over volume. Each course goes through rigorous verification against authoritative sources (specs, RFCs, source code). We'd rather have one excellent course than a hundred mediocre ones. More courses are being developed.
Underlayer offers multiple review modes: Quick Review (fast refresh), Deep Review (thorough testing), Weakness Focus (targeted at weak concepts), and more. Choose based on your available time and learning goals.
Yes. Visit Settings to export your progress, review history, and analytics as JSON. Your data belongs to you.
Open an issue on GitHub. Include as much detail as possible: what you were doing, what you expected, and what actually happened.
Underlayer is built with the Chemical programming language, uses SQLite (or Turso) for the database, and compiles course content to static HTML. See the About page for more details.
Yes. Underlayer does not track you, sell your data, or use analytics services. Your progress is stored in your account and nowhere else. You can delete your account and all data at any time from Settings.