What is a commonplace book
A commonplace book is one notebook of other people's lines, collected as items so they can return.
You type what is a commonplace book because the phrase keeps showing up, and the results do not agree. Some point to a second brain. Some point to a diary with quotations mixed in. You want the simple answer.
A commonplace book is one notebook of other people's lines. You collect them so they can return. That is the whole definition.
The unit is an item. The item is the text, its source, and the date you collected it. You are its keeper, not its author. A short annotation about why it stayed is enough. Skip the annotation if the line is enough.
It is not a journal of your day. It is not a vault of files. It is not an index you build before you have anything to keep. The book grows from what you actually collect.
The point is recollection. Later, once you have half-forgotten the line, the item returns. Collecting without return is only storage. Return without a source is only a blur.
Paper or a phone does not change the job. The medium is not the definition. The practice is: other people's words, kept as items, ready to come back.
If that is the search, continue here: How to keep a commonplace book https://opuss.com/blog/keep
Opuss is that notebook, for iPhone. Free on the App Store. https://apps.apple.com/app/id6785198553
You collect an item. You can add an annotation. Later it returns as a recollection.
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The letter
Read issue 1. Ten we kept this week, and how to keep your own. Or start with how to keep.