The Melvil Decimal System is named after Melvil Dewey, whose Dewey Decimal System is used by
libraries around the world.
The system is "decimal" in that it is entirely composed of numbers. Each level of the classification is a number from 0 to 9. This makes MDS/DDC easy to understand and manipulate. It is the choice of most US public and school libraries, and is used around the world.
Although Dewey invented his system in 1876, recent editions of his system are in copyright. LibraryThing's Melvil Decimal System is based on the classification work of libraries around the world, whose assignments are not copyrightable. The "schedules" (the words that describe the numbers) come from a pre-copyright edition
of his system, John Mark Ockerbloom's Free Decimal System, and member contributions.