A hierarchy is a system in which things are arranged in levels, with some things above others. It’s like an inverted tree structure. It may help to think of it as a family tree. So what makes archival catalogues hierarchical?
The collection, or 'fonds' level is the highest level of description in an archival catalogue. It tends to relate to a particular person or institution. At this level, you get an overview of the entire collection. By following the hierarchy down to the lower levels, you get more detailed descriptions.
The next level down is usually the 'series' level. This relates to particular types of activities, for example, in the papers of authors there are often separate series for prose, poetry and correspondence.
Then you get to the lower levels, usually called 'sub-series' or 'files'. These levels tend to relate to particular events, or instances of the activity the series relates to. A file is often the lowest level of description, although sometimes archivists list every single item.
For example, there might be a file relating to a particular correspondent within a series of correspondence. These files can contain over a hundred letters, in which case cataloguing to 'item' level may not be possible and the file is the lowest level in that branch of the hierarchy.
But another file relating to a different correspondent might contain only a few letters in which case the letters can be catalogued individually, and that file is not the lowest level in that branch of the hierarchy – it has several children which are descriptions of the individual letters. The letters are the lowest level of that branch, not the file.
An 'item' can vary in size but it is intellectually indivisible, for example a letter comprising several pages is still considered an item because all of the pages are part of one letter.
There aren’t a set number of levels, as you will see in the next page.
When cataloguing, archivists try to avoid repetition. So if some part of the description applied to every file in a series, that information might be included in the series level description instead of the lower level descriptions.