The reader

The lesson is the middle column. This page is about everything around it — the parts that help you find your place, keep it, and come back to it.

The library

The landing page is the library: every book as a card, grouped by subject. Signed in or not, each card shows how much of the book you have read, and a pick up where you left off card above the grid takes you back to the last lesson you opened.

If you are on the reader list of a private book, it appears in its own group at the top of the grid once you sign in. Nobody else sees it, signed in or not.

Above the library, the tour rotates through four slides of what a lesson can do. It pauses when you hover over it.

Finding your way in a book

On a wide screen a lesson has three columns: the book's contents on the left, the lesson in the middle, and an outline of the page on the right.

• The sidebar has three faces — expanded, a compact rail, or hidden. Your choice is remembered. Finished lessons carry a ✓, and a bar at the top shows how far through the book you are.
• The outline on the right lists the page's headings and highlights the one you are reading. A thin bar across the top of the page shows how far down you are; a button appears to jump back to the top.
• On a narrow screen the contents move into a drawer. Open it with the round button in the corner, or the Browse pill on desktop; close it with Escape, a tap outside, or by choosing a lesson.
• Previous and next sit at the bottom of every lesson.

⌘K: search everything

Press ⌘K (Ctrl+K on Windows and Linux), or click the search box in the header, from any page.

Two searches run as you type. The first is instant and matches titles and the chapter path. The second asks the server, which has read the prose of every book in the library — so a concept explained at length but never named in a heading still turns up, with the sentence that matched shown under the result. Blog posts are searched too. If the server search is slow or you are offline, you still get the instant results.

Reading preferences

The Aa button opens the reading preferences:

They are stored in your browser and apply to every lesson.

Mark as read

At the end of every prose lesson there is a Mark as read switch. Nothing marks a page for you — scrolling to the bottom is not the same as reading it, so the decision is yours. A problem is marked for you when the judge accepts your solution.

Signed out, ticks live in this browser. Signed in, they are saved to your account and follow you to any device. /account has a Reset progress button that clears ticks and nothing else.

Suggest an edit

Every lesson has a Suggest an edit link, and every D2 figure has an Edit pill. If you are signed in and on the content-editor list, the link opens the in-app editor for that lesson; otherwise it stays visible but disabled, and hovering it tells you how to ask for access. If editing is switched off on a deployment, the link is not there at all.

What happens after you submit — a pull request, reviewed by a maintainer — is in Accounts and contributing.