del.icio.us allows you to create tag intersections. A tag intersection is the list of items tagged with both A and B. You can use tag intersections to see items that share common tags.
Example: http://del.icio.us/jameselee/cool+video displays all the bookmarks I've tagged both "cool" and "video". Anything tagged "video", but not "cool", won't show up in the intersection.
This is Wikipedia's example diagram of an intersection in this context:
Tag intersections are a type of what I call a "tagset", just as in mathematics, an intersection is a type of set.
Tag intersections let you filter information to create specific views and see relationships.
- Create useful lists - of "movies+toWatch", "photography+tips", "solutions+didWork", "books+toRead", "places+toVisit", "places+didVisit", "restaurants+toTry", etc.
- Create topic-specific feeds - People can create and subscribe to specific (RSS) feeds of your tag intersections "movies+didLike", "restaurants+recommendations", "computer+tips", etc. You can do this with any del.icio.us tags; your own, others', all. This is a good way to follow what people are bookmarking on specific topics.
You can use tag intersections to filter your own tags: http://del.icio.us/jameselee/ideas+tagging or all tags in del.icio.us http://del.icio.us/tag/ideas+tagging. (Note that some del.icio.us-wide tag intersections, especially for popular tags such as "cool" and "video" may cause del.icio.us to return a blank page, possibly due to the load required to filter on so many results.)
Some guidance on using tags
Use many single-word tags instead of one multi-word tag. Tag intersections enable you to narrow your focus when you want to, so you can use more general single-word tags (and most likely end up with fewer overall tags). The value of intersections increases as you use more tags to describe your bookmarks, something del.icio.us makes it really easy to do.
Compare the 2 tags, "softwaretools" and "hardwaretools" vs. the 4 tags, "software tools" and "hardware tools". The former uses fewer tags, but they are unnecessarily specific. The latter case uses more tags, but each can apply to a wider variety of bookmarks. This approach gives you the option of looking at everything tagged "tools", to see hardware and software tools, as well as things you've tagged "gardening tools" and "car tools".
There are three ways to create tag intersections:
- Manually type the tags into the address bar of your browser, e.g. http://del.icio.us/tag/A+B
- Manually type A+B into the breadcrumbs shortcuts on any del.icio.us page.
- "Build" them in the del.icio.us interface (which is very powerful, and underrated) by adding related tags:
- Look on the right side of your del.icio.us page, to see a list of your tags, which you can display as a list or a cloud. (I recommend a cloud if you have a large number of tags, though list view is useful for seeing the number of items associated with each tag, which helps in this case.)
- Click on the first tag you want to use to build your intersection; in this example, we'll use "learning". (The order is not important, so "learning+reference" is the same as "reference+learning", but order can help tag intersections make sense to humans.)
- Look at the list of related tags that appears next to your list and click on the "+" beside the next tag you want to add to the intersection. (This list appears only if your first tag contains at least one bookmark that has at least one other tag.):
- You can narrow your filter by adding more tags to the intersection, or broaden it by removing tags:
- If you frequently use a tag intersection, create a saved search - Try entering your intersection, then tagging the del.icio.us page that displays the results of the tag intersection filter. (Tagging del.icio.us pages is an idea with a lot of potential, about which I plan to write soon.) See myTagsets for examples.
Note that many of these concepts apply -- perhaps with some variations -- to any service/software that supports tag intersections. Flickr
is one such service, and there seems to be some discussion on the topic in the Flickr forums