While Molfiles are two dimensional Chemical Tables that cannot be canonicalized because of the embedded atom coordinates, line notations encode basic molecular structural information into a one-dimensional string of characters that makes chemical structures accessible for machine learning. The most common line notations are SMILES and InChI. The Simplified Molecular Input Line Entry System (SMILES) is very common in computational chemistry. "SMILES is a true language, albeit with a simple vocabulary (atom and bond symbols) and only a few grammar rules. SMILES representations of structure can in turn be used as "words" in the vocabulary of other languages designed for storage of chemical information (information about chemicals) and chemical intelligence (information about chemistry)." The "words" consist of characters representing atoms according to the symbols of the chemical elements, bond types connecting each of the atoms, and numbers indicating ring closures. Each chemical structure may be represented by multiple equivalent SMILES strings; canonical SMILES are supposed to offer a unique representation for each compound. SMILES are human-readable and machine-digestible but suffer from many shortcomings such as the existence of multiple SMILES "dialects", vendor specific canonicalization algorithms for SMILES, the non-ideal treatment of tautomers and inorganic compounds, and the limited functionality to represent non tetrahedral stereochemistry. The IUPAC International Chemical Identifier (InChI) is a structure-based canonical chemical identifier, i.e. each chemical structure can only be represented by one InChI specific for this structure and every InChI identifies just one molecule. Hereby, identical structures can be recognised as such, and chemical data of structures can be linked e.g. in repositories, in databases, or on the internet. InChI encodes chemical structures into a layered line notation. Unlike SMILES, InChI only represents the pure connectivity of a molecule, but not the bond order between the atoms.