[Update: Now that Aperture 1.5 is out, keywords have changed significantly. This article was written for keyword management in 1.1]

Keywords in Aperture are applied in a hierarchy. It's not obvious that this happens, and I have seen several postings that say it does not work that way, but I have tried it, and it really does. What is confusing is that the hierarchy is hidden almost everywhere.

What do I mean by a hierarchy? The keyword HUD (Shift H) shows a list of keywords grouped into two types of keywords: plain keywords and container keywords. For instance, Person can contain Adult and Child. All three are keywords and will be applied to images and appear in the displayed metadata.

If I apply Child to an image, then Person is applied for me, but it does not appear in the keyword list in the overlays or in the simple keyword lists. But it is really there, and you can see it two ways. Bring up the information pane with I and click on the image that Child was applied to. Select Metadata: General and select Keywords at the bottom. In the list there will be Child (Person) listed. The second way is to type Person into the search box. The image you applied Child to will be found.

You can go further, of course. Make sub-keywords of Child: Fred, Jim, Sally and apply those to the images. Now the keywords listed in the Metadata: General pane say Fred (Person > Child). And Fred will be found by any search on Person or Child or Fred.

Now what happens when Fred grows up and becomes an adult? If Fred is moved to the Adult keyword (or added to it), then new images will be tagged with Fred (Person > Adult) and searches for Child will not find him. Existing image keywords are not modified.

There's more. I can have the keyword Fred in other hierarchies too, such as Soccer. Now if I add Fred from the Soccer hierarchy the metadata list will say Fred (Soccer) as well as Fred (Person > Child). The normal display of keywords will just list Fred once, but in the Metadata: General keyword list it will be listed twice with the hierarchy listed each time.

So when Fred becomes an adult I take the trouble to go back and add all the Fred keywords again, then they will be tagged twice with Fred, once as Fred (Person > Child) and once and Fred (Person > Adult).

Here is more confusion. If I search by bringing up the filter (to the left of the search box) and enable keyword searching, the only keywords that are listed are the ones that have actually been added. So Fred, Jim, Sally are there, since I applied them to the images. But Person, Adult, Child, Soccer are not. If fact if there are several Fred listings, they are differentiated by being listed as Fred (Soccer) and Fred (Child). But only to one level; Person is not in the brackets. And if you export with metadata, only the keywords you actually added are exported.

There are really two classes of keywords then, those added using the keyword HUD that include hierarchy information, and those that are just typed in to the keyword field (or presumably added in other ways).