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Sources & methodology
Our library is human-curated and openly sourced. Every card is drawn from a trusted, openly licensed source — or written by our own editorial team — and the source is shown on the card so you can check it. Here is exactly where the content comes from and the research the app is built on.
Last reviewed 2026-06-20.
Each source below is openly licensed or public domain, and we list the license we use it under.
Structured facts — capitals, elements, dates, definitions — that anchor knowledge cards to a checkable public record.
Peer-reviewed, openly licensed college textbooks behind many of our science and math cards.
A large open collection of example sentences with translations — the backbone of our language cards.
Government chemistry data we draw on for compound, element, and reaction cards.
Decks contributed by educators and learners, used with permission and credited to their authors.
Cards written and reviewed by Koydo's editorial team where no good open source exists.
How the library is curated
Our editorial team — Koydo Lyceum — selects open sources, writes original cards where no good source exists, and reviews the library for accuracy. Nothing is bulk-generated. Where a card uses an outside source, that source and its license are credited in the app.
Our scheduling and study design follow well-established findings in cognitive science. You can see the spacing idea in action in our free spaced repetition calculator.
The landmark study on the testing effect: recalling an answer beats re-reading it. Why our cards ask you to recall, not just review.
A synthesis of the spacing effect across many studies: spreading reviews out beats cramming. Why reviews are scheduled over time.
The open-source, evidence-based scheduling model whose ideas inform when each card comes back — so timing is handled for you.
No. The library is human-curated. Cards are drawn from openly licensed sources or written by Koydo's editorial team, and each card credits its source — none is bulk-generated by AI.
From openly licensed and public-domain sources — including Wikidata, OpenStax, Tatoeba, and PubChem — plus community shared decks used with permission and Koydo's own authored material. Each source's license is listed on this page.
Yes. The source is shown right on the card while you study, so you can verify where any fact came from.
Because decades of cognitive-science research show that spacing reviews out and recalling answers (rather than re-reading them) leads to stronger long-term memory. We build that timing in so you do not have to manage it.
It was last reviewed on 2026-06-20. We update it whenever our sources or methodology change.