September's project in my one-project-a-month challenge is Lemmon, a flashcard app that started with a very ordinary frustration: I did not enjoy using Anki Web.
Flashcards fit the way I want to study, but creating and managing them felt more awkward than I wanted. Before I could practise a definition or a mathematical idea, I had to spend time getting the card itself right. I wanted that preparation to feel easier, and I wanted an interface I would enjoy coming back to.
That became the starting point for Lemmon: a small learning tool shaped around how I actually study.
Less effort between an idea and a card
The first priority was writing cards. A plain question and answer work for some subjects, but mathematical study often needs more: notation, a diagram, or formatting that makes the important part easy to see.
Lemmon lets me write both sides of a card with rich formatting, including bold, italic, underline, and text color. Images can be uploaded, pasted, or dropped into the editor. That means a useful diagram can sit beside an explanation instead of forcing everything into text.
Equations deserved particular attention. I wanted to insert a formula where it belongs in a sentence, or give it its own line when it needs more space. The editor supports both inline and display equations, with a visual math input, an editable LaTeX source view, and searchable notation templates. Clicking an existing equation opens it for editing again.
For me, the useful part is keeping that whole process inside the card editor. If I am thinking about a definition, I want to keep thinking about the definition while I write it down.
Organization that grows with the material
A collection of cards quickly needs structure. Lemmon uses decks to group material and tags to describe individual cards. Existing tags are suggested within each deck, and a new tag is created when the card is saved.
That is a small detail, but it reflects the workflow I wanted: I can start creating useful cards and let the organization develop as the material grows. I do not need to settle on a complete tagging system before writing the first question.
I also wanted to make it easier to inspect what I had written. Card previews help me browse a deck before a study session. Another goal was a table with questions and answers side by side, so checking the content would be a simple reading task. Creating cards, checking their wording, and testing myself are different activities, and each needs a comfortable view of the same material.
A review loop that stays simple
During a review, Lemmon shows a question, lets me reveal the answer, and asks me to rate it: Again, Hard, Good, or Easy. Those ratings adjust when the card becomes due again. There is also a practice mode for going through all the cards without changing their review schedule.
The app can be added to a phone's home screen and supports offline study after the decks have been loaded online. Offline reviews are saved on the device and synchronized when I reconnect. Creating and editing cards still require a connection.
These choices support the same intention as the editor: make it easier to sit down and study, with fewer interruptions from the tool.
Building the experience
Lemmon is built with Next.js, React, and TypeScript, with Convex for the backend and Better Auth for accounts. Tiptap provides the rich text editor, MathLive handles visual equation entry, KaTeX renders the mathematics, and Vercel Blob stores uploaded images. The app is deployed on Vercel.
An important design decision was to carry the same rich content from writing a card into previews and study sessions. A formula needs to remain readable wherever the card appears, and editing it should feel like continuing to write the card.
I wanted the surrounding interface to stay minimal and pleasant. Decks, cards, and the next review should be easy to find, while the material itself gets most of the attention.
Why Lemmon?
The name combines lemon and lemma: a playful identity with a connection to mathematical study. It fits a project that I wanted to feel light and approachable while still being useful for technical material.
After Plate Atlas in July and VibeType and Easy Meet in August, Lemmon continues a theme in this monthly challenge: taking a personal need seriously enough to build something around it. This time, the reward is having a learning tool whose features I can shape as my study habits change.