I have been playing with NHibernate again recently.
Unless you have been on Mars for the last 6 months you will know that Microsoft has reignited the interest in ORM generally with their recent entry into the foray (in the form of the ADO.Net Entity Framework); and this has prompted me to get out, dust off and polish up my NHibernate knowledge. Whilst doing this I came across this page which is a nice succinct page written by Tom Sedge explaining the HBM syntax for various relationships.