Pinning lets you create some nested tags even when no files live at the tags intersection.

By default, the directory /studies/climate will only exist if there are files tagged with both studies and climate. This is because tagdirs are tag intersections, meaning that if there are no files with both of those tags, that specific directory path won’t exist. Having it be this way is useful for navigating directories, because you will only see sub-tagdirs that are relevant to your current tagdir. However, it is not so useful when populating the tagdirs for the first time, because you may want to navigate to the nested tagdirs before linking anything into them. To alleviate this problem, we use tagdir pinning.

Pinning is creating a temporary “pin” in a tagpath, so that you can navigate to it without requiring actual files to exist inside of it. Without tagdir pinning, you could not browse to /studies/climate unless there already existed files tagged with studies and climate.