Tags allow you to describe what kind of thing a Rem is, in a way that cuts across all documents and hierarchies in your Knowledge Base. For instance, you might have an “Author” tag you apply to Rems representing authors you like, or an “Urgent” tag which you place on todo items that you need to complete very soon. You can attach a tag to a Rem by putting your cursor within the Rem you want to tag, then typing ## and the tag's name; or by copying the tag, then pasting it over the Rem you want to tag.

Tags, References, and Portals are all ways of relating Rems to each other. If you're not sure which one to choose, check out What's the difference between References, Tags, and Portals?

Tags are closely integrated with properties, templates, and advanced tables. For a quick overview of how these all connect, see Tags, Properties, Tables, and Templates in Five Minutes.

Because everything is a Rem, a tag is just a plain old Rem that has some additional metadata attached to it. This means you can use any Rem, located anywhere in your knowledge base, to tag any other Rem! For instance, you might want to take notes on what a Chemical Element is (the history of the idea, how elements are organized in the periodic table, and so on), making Chemical Element into a heading or Concept and adding notes beneath it. But you might also want to tag each element you take notes on with Chemical Element. Since Chemical Element is a Rem, you can do both, and the Chemical Element Rem will keep track of both those notes and a list of all the other elements you tagged.

To apply a tag to a Rem, type # twice in your editor. Then select an existing tag or type a new name and choose Create Rem (or press Ctrl+Enter, or Cmd+Return on a Mac).

Since tags are just Rems, you can theoretically use any Rem in your knowledge base as a tag. To make it easier to find the Rems that you’re likely to want to tag something with, the ## tag search menu starts by only showing Rems that:

If you want to start using some other Rem as a tag, just choose the Show Non-Tags option in the list to expand the search to all Rems:
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Since, again, tags are just Rems, you can access them in the main editor, rename them by changing their text, move them into hierarchies, etc.

At the bottom of a tag's page, all the Rems it's tagging will be listed. Here, we open up the Urgent tag by clicking on it, look at the list of Rems it's tagging, and change its name to Super Urgent.

Tip: If you highlight a tag's Rem using /red, /orange, etc., the tag will appear in that color. Here's the to-do list above after highlighting the Super Urgent Rem in red:
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Sometimes it's helpful to show only the Rems in some document that have a particular tag, hiding everything else. For instance, you can hide all to-do items except those that are urgent. There are two ways to do this:

You can use a Book tag to collect all your book notes in one place across your Knowledge Base. Because tags integrate with templates and advanced tables, any Rem you tag with Book will automatically appear in a table you set up as a reading list - along with any properties you've defined, like author, genre, or reading status.

While reviewing flashcards, you might find certain cards consistently trip you up. You can create a Hard Card tag and apply it to those cards as you go, then later practice that specific set to focus your study session. Since the tag is just a Rem, you can delete it when you no longer need it, and all instances of it disappear automatically.

Tags are useful whenever you want a secondary way to group Rems that live in different documents. For example, you might tag daily notes and project Rems with Currently Active Project to get a single view of everything related to that project, regardless of where those notes are stored. Combined with search portals, this lets you pull together content from across your Knowledge Base without moving or duplicating anything.