Hi Josef

Apologies for the previous premature email; it seems I cannot read.

:-)

The apparent contradiction I mentioned was the result of my conflating 'callback' with 'addCallback'.

Right.

However, I guess I'd still appreciate some discussion on the nuances of these two functions.

It's just a trivial non-real-world example. The use of callback is to supply a return value, which is passed to the function added with addCallback (and similarly with errback). In a non-trivial example, some other code (associated with the function that returned the deferred) would be what called callback (or errback). The example just shows all the code in one place. Does that make sense? Terry