This part of ISO 17450 provides a model for geometrical specification and verification and defines the corresponding concepts. It also explains the mathematical basis of the concepts associated with the model and defines general terms for geometrical features of workpieces. This part of ISO 17450 defines the fundamental concepts for the GPS system in order to: — provide nonambiguous GPS language to be used in design, manufacturing and verification, — identify features, characteristics and rules to provide the basis for specifications, — provide a complete symbology language to indicate GPS specifications, — provide simplified symbology by defining default rules, and — provide consistent rules for verification.