Blades are defined as being flakes that are at least twice as long as they are wide and that have parallel or subparallel sides and at least two ridges on the dorsal (outer) side. Additionally, a tool must be part of an intentional blade industry in order to properly be considered a blade; tools which show the characteristics of blades through variation but are not intentionally produced with those characteristics are not considered true blades. Blades became the favoured technology of the Upper Palaeolithic era, although they are occasionally found in earlier periods. A soft punch or hammerstone is necessary in creating a blade and their long sharp edges made them useful for a variety of purposes. They were often worked to create scrapers or burins.