John Locke's method
Locke's index of two pages: first letter, next vowel, and a way back to the item.
Locke published his method of a commonplace book because a friend asked him to. He had used it for five-and-twenty years and thought it too slight for print, until experience among friends forced his hand. The English text sits in his Works; the 1823 pages are open on Wikisource, and the 1706 London pamphlet is on the Internet Archive.
He begins with an index of two facing pages. Twenty letters of the alphabet, each with the five vowels beneath. Three letters he omits, K, Y, and W, as covered by C, I, and U. Every head he will ever use is classed by its first letter and the vowel that follows.
When he wants to collect under a head, Epistola, say, he looks up E i in the index. If a page number is already there, he writes under that head on that opening. If not, he takes the next blank verso, writes the page into the index, and claims the two pages that face each other for that class alone. The head sits in the margin in large letters so the eye can find it without turning the leaf.
The point is not filing. It is finding again. The index is two pages; the book can be as thick as he likes. A class owns a spread so every head in that class is visible at once.
That is a keeper's tool: collect the item under a head that matters, leave a way back, and trust the return.
How to keep a commonplace book · The letter · 1 · Opuss on the App Store
Sources: John Locke, "A new Method of a Common-Place-Book," in The Works of John Locke, vol. 3 (1823), via Wikisource. First English pamphlet, London, 1706: Internet Archive.
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