I'm sure everyone knows what an XML declaration is. If you don't, here is an example to refresh your memory:

<?xml version="1.0" ?>

I've frequently referred to this as an XML Processing Instruction. Thanks to one of our readers, however, I've now been set straight. As you may recall, a Processing Instruction (PI) is the XML specifications concession that things are not always neat and tidy in the real world. It gives document authors an opportunity to issue instructions to a particular application that may process the document. The PI takes the form

<? PITarget 'stuff' ?>

where PITarget is a name that identifies the application, and 'stuff' can be pretty much anything and is required to be passed through to the application. While the XML declaration uses the same notation, it is explicitly called out in the specification as reserved for standardization and is not a Processing Instruction.