This document establishes a system of designation for thermoplastic polyester (TP) material, which can be used as the basis for specifications. It covers polyester homopolymers for moulding and extrusion based on poly(ethylene terephthalate) (PET), poly(butylene terephthalate) (PBT), poly(cyclohexylenedimethylene terephthalate) (PCT), poly(ethylene naphthalate) (PEN), poly(butylene naphthalates) (PBN) and other TP-types and copolyesters of various compositions for moulding and extrusion. The types of thermoplastic polyester are differentiated from each other by a classification system based on appropriate levels of the designatory properties: a) viscosity number; b) tensile modulus of elasticity; and on information about the intended application and/or method of processing, important properties, additives, colorants, fillers and reinforcing materials. This designation system is applicable to thermoplastic polyester homopolymers and copolymers. It applies to materials ready for normal use in the form of powder, granules or pellets, unmodified or modified by colorants, fillers and other additives. This document does not apply to the saturated polyester/ester and polyether/ester thermoplastic elastomers covered by ISO 20029. It is not intended to imply that materials having the same designation give necessarily the same performance. This document does not provide engineering data, performance data or data on processing conditions which can be required to specify a material. If such additional properties are required, they are intended to be determined in accordance with the test methods specified in ISO 20028-2, if suitable. In order to designate a thermoplastic polyester material to meet particular specifications, the requirements are to be given in data block 5 (see 4.1).