[Forgive me if this duplicates. My mailer claims I sent it, but I never got it and I don't see it in the archives]

Gregor Hagedorn writes:

It is difficult to add information about obovate or length. Noel already noted that we may be running into this "properties of properties" problem.

General question: Is there any reason why attributes exist at all? What is the advantage of

<leaf shape="obovate" length_mm="31"/>

over

<leaf> <shape>obovate</shape> <length>31</length> <MeasurementUnit>mm</MeasurementUnit>=" </leaf>

Popular wisdom often asserts there is no difference, but there are important differences for database work which are less so (except 1 below) for documents (hence the popular wisdom). These things are true of attributes in XML but not necessarily of elements (the tags + stuff between them) depending on the grammar defining the vocabulary:

1. Attributes are order independent

2. There can only be one attribute,value pair for each attribute.

3. With DTD's (which are the wrong thing to consider here but that is another story), you can enumerate the permitted values of attributes. You can work around this for elements but it may be pointless, verbose, and fragile, especially because of the order dependence of elements.

4. (Subtle and technical)References to XML objects essentially must appear as attributes. This is the only way you can get non-tree graph structure in XML documents, and also is the way you would make a data value conveniently be another object. So if "obvate" were actually some complex object that held more information, you would probably find it named as the value of an "idref", not as text data. While specimen and species documents may prove to representable by trees, keys probably will not be.