A chemical file format is a type of data file which is used specifically to depicting molecular data. One of the most widely used is the chemical table file format, which is similar to Structure Data Format (SDF) files. They are text files that represent multiple chemical structure records and associated data fields. The XYZ file format is a simple format that usually gives the number of atoms in the first line, a comment on the second, followed by a number of lines with atomic symbols (or atomic numbers) and cartesian coordinates. The Protein Data Bank Format is commonly used for proteins but is also used for other types of molecules. There are many other types which are detailed below. Various software systems are available to convert from one format to another. Distinguishing formats Chemical information is usually provided as files or streams and many formats have been created, with varying degrees of documentation. The format is indicated in three ways:(see ) file extension (usually 3 letters). This is widely used, but fragile as common suffixes such as ".mol" and ".dat" are used by many systems, including non-chemical ones. self-describing files where the format information is included in the file. Examples are CIF and CML. chemical/MIME type added by a chemically-aware server. Chemical Markup Language Chemical Markup Language (CML) is an open standard for representing molecular and other chemical data. The open source project includes XML Schema, source code for parsing and working with CML data, and an active community. The articles Tools for Working with Chemical Markup Language and XML for Chemistry and Biosciences discusses CML in more detail. CML data files are accepted by many tools, including JChemPaint, Jmol, XDrawChem and MarvinView. Protein Data Bank Format The Protein Data Bank Format is commonly used for proteins but it can be used for other types of molecules as well. It was originally designed as, and continues to be, a fixed-column-width format and thus officially has a built-in maximum number of atoms, of residues, and of chains; this resulted in splitting very large structures such as ribosomes into multiple files. However, many tools can read files that exceed those limits. For example, the E. coli 70S ribosome was represented as 4 PDB files in 2009: 3I1M, 3I1N, 3I1O and 3I1P. In 2014 they were consolidated into a single file, 4V6C. Some PDB files contain an optional section describing atom connectivity