One only comes across an empirical formula in an examination question as an intermediary step in the calculation of the molecular formula from the elemental percentage composition. In indexes, the compounds are first listed in order of increasing carbon number and, for molecules with the same number of carbons, in increasing hydrogen number. The notations fall into two categories: two-dimensional and three-dimensional. The former type portrays the molecule as though it was flat, and leaves unresolved any explicit information about the three-dimensional structure. The use of a third symbol may also be employed in order to distinguish between the origins of electrons from different atoms of the same element, for example the two carbon atoms in ethane. In Line notation the chemical symbols are written on one line using the normal atomic symbols and relevant subscripts or superscripts. The carbon of interest is represented by a dot in the centre of the circle and is nearer the reader.