The .ppm file extension is used for Portable PixMap format files and is intended to be an intermediate format that can be used in the development and conversion of other file formats. This format was specifically designed so that it could easily be exchanged between various platforms. It is very simple but not very storage efficient. Before the PPM format was introduced, Jef Poskanzer invented the PBM format in the year 1980 for transmission of monochrome bitmaps as ASCII text in email messages. He also developed a tool library for easy conversion of the PBM files to other graphical formats and vice versa. This library was called Pbmplus and was released in 1988. Later Poskanzer developed the PPM format along with the PGM format and the requisite tools that were added to Pbmplus.