Software versioning is the process of assigning unique version names or unique version numbers to unique states of computer software. A commonly adopted scheme for version numbers is known as semantic versioning (SemVer), which comprises a three-part version number (Major.Minor.Patch), an optional prerelease tag, and an optional build meta tag. A fourth number may also be used to denote the software build, as was the case for Adobe Flash. Some companies also rely on the build date, in a system known as calendar versioning, and letters and other characters, such as Lotus 1-2-3 Release 1a.