Intentional and Functional Affordances

In Costall and Dreier's new book (2008), Doing things with Things, there is a neat distinction drawn between intentional and functional affordances.

Functional affordances are affordances which arise out of the properties of the object, event, interaction, whereas intentional affordances are affordances which arise primarily out of the intentions of the user, the person interacting with the object, event, etc.

This provides a useful spectrum for description and analysis, and I suppose one could envisage 'purely' functional affordances at one end of the spectrum, and 'purely' intentional ones at the other end.