This European Prestandard gives tables of preferred dimensions in millimeters and masses per unit length in kilogrammes per metre of plain and seamless and welded steel tubes. It covers two groups of tubes: -- group 1: tubes for general purposes (e.g. pressure, structural and mechanical application; see tables 1 and 2) -- group 2: tubes for special requirements regarding tolerances and surface finish, called "precision tubes" in the following (see table 3) Relevant subcommittees of ECISS and relevant Technical Committees of CEN should select these dimensions for their tubes. The outside diameters are classified into three series for group 1 and two series for group 2 reflecting the availability of other components of a pipe line (see clause 2). This classification of outside diameters into different series and of preferred wall thickness indicates the range of manufactured steel tube available today. The masses given in the Prestandard are calculated from the nominal diameter and thickness. To calculate the masses per unit length of tubes with different dimensions from those given in the tables, the formula in clause 3 should be used. It should be noted that it may not be possible in all cases to supply series 2 and 3 tube sizes for which the values are given in the tables.