The frieze of the Doric entablature is split into triglyphs and metopes (a unit consisting of three vertical bars separated by grooves). The Greek forms of the Doric order lack a personal foundation. Instead, they are based on the number and size of the blocks with which they are built. The Doric order is so called because it was originally used as part of a building decorated with sculptures representing trees or plants. Trees were important in ancient Greece: both poets and painters enjoyed imitating their shapes in sculpture. The first examples of this order that have survived date from about 500 B.C.; but it was probably invented much earlier. In classical architecture, the term "Doric" usually refers to the order given specific content with regard to dimensions and style of construction. Thus, Doric architecture is that which uses Doric materials and techniques for its construction. By contrast, Ionic and Corinthian orders are considered styles of decoration rather than actual buildings types. However, some architects have created versions of these orders that use components similar to those found in traditional Doric buildings. These creations are not considered authentic representations of the Ionic or Corinthian orders but rather examples of modified Doric structures. It is also possible to build hybrid versions of Doric and other related orders such as Egyptian or Persian.