Suppose you want to change the way addresses are handled in your instance; your customer needs a third field in addition to Odoo's two fields (street and street2) to store addresses. You can certainly write your own add-on to add a field to res.partner, but the issue is a bit trickier than it seems if you want the address to be properly formatted on invoices. Fortunately, someone on a mailing list tells you about the partner_address_street3 add-on, which is maintained by the OCA as part of the partner-contact project.

The paths that are used in this recipe reflect the layout that was proposed in the Standardizing your instance directory layout recipe.