Bill Wake devised the acronym INVEST as a reminder of the properties of good stories: Stories ideally describe non-overlapping bits of functionality, possible to implement in any order. Stories have flexibility; there's room for a collaborative definition of success. Stories' value makes sense from a customer or end user's perspective. They're not described in terms of a developer's tasks. The team can estimate the difficulty of the story (at least to a rough level). The story is small enough for its purpose. For implementation, it can be completed in less than an iteration. For longer term planning, the story is larger. The customer, developers, and testers can agree on what the story means precisely enough that tests can be created.