Bracketed and indented keys

As mentioned before, the identification keys published by taxonomists and used by all manner of end-users of taxonomic literature to identify organisms to species are traditionally analytic, single entry and dichotomous. That means they consist of series of nested questions that have to be answered one after the other in a fixed order to arrive at the correct species name.

In technical terminology, the individual questions are called couplets, and the two alternative answers of each are called leads. A good key should have leads describing a limited number (perhaps ca. three) of clear, distinctive, and easily understood characters.

However, even for the exact same content there are two different ways of formatting these keys. The first option is a bracketed key. It looks as follows:

1     Leaves 0.5-2.0 cm long ... 2
      Leaves 3.0-10.0 cm long ... 3
2(1)  Corolla red ... Planta australis
      Corolla white or yellow ... Planta latifolia
3(1)  Fruit a capsule ... 4
      Fruit a berry ... Planta vulgaris
4(3)  Flowers solitary ... Planta palustris
      Flowers in axillary clusters ... Planta debilis

Here the two leads of the same couplet are always directly below each other. This has the advantage that it is very easy to compare the two alternatives, but on the other side the couplets need to be numbered in some way so that we know where we have to go next. In larger keys, it may also be harder to find the next or, if back-tracking, the previous couplet because one has to jump around so much.

In the above case, each couplet except the first also has a number in brackets directly after its own. This indicates where the user had to be coming from to arrive at this place, making back-tracking in large keys easier. But of course not all large keys have them.

The second option is called an indented key. Here is exactly the same key as above in an indented form:

Leaves 0.5-2.0 cm long
   Corolla red ... Planta australis
   Corolla white or yellow ... Planta latifolia
Leaves 3.0-10.0 cm long
   Fruit a capsule
      Flowers solitary ... Planta palustris
      Flowers in axillary clusters ... Planta debilis
   Fruit a berry ... Planta vulgaris

In this case, the follow-up, nested couplets are directly after the lead that, well, leads to them. On the one hand, this makes navigation easier, and couplet numbers are not strictly necessary. On the other hand, it is harder to compare the two leads of each couplet. Worse, indentation does not work very well to indicate what couplets belong together if the key is long enough to spread over several pages. Imagine reading trying to find the second lead two or three pages down the book and then deciding that the first one was right after all! Unfortunately, some authors do not seem to see a problem with that.

Reading between the lines, it may have come across that I prefer bracketed keys, and that is indeed the case. It seems that bracketed keys are more popular in continental Europe while indented ones are traditionally more popular in Anglo-Saxon countries. Be that as it may, with the increased availability of digital keys the bracketed variant appears to be on the rise.

Of course internet and other computer based keys allow designs that go far beyond the limitations of analytic single entry keys. But the latter are still widely used, and they do have their advantages. At a conference two years ago some colleagues pointed out that working through a single entry key makes it much easier to learn about the whole diversity of and character distribution in the group, whereas using a table-based key is much more of a 'black box' experience.

The thing is, when single entry keys are used on a digital medium, there is pretty much no point in separating the two leads, especially if the key is supposed to work on small displays. In effect digital keys therefore tend to be increasingly bracketed.