Abstract:7.1 Introduction7.2 Fibres and yarns: definitions and factors affecting quality7.3 Fibrous assemblies: volume, density and mass7.4 Yarn parameters: fineness, density, diameter and twist7.5 Modelling of internal yarn geometry: helical yarn model7.6 Stress–strain relations7.7 The relationship between yarn count, twist, packing density and diameter7.8 A simple mechanical model of yarn properties7.9 Mechanics of parallel fibre bundles7.10 Examples of models of fibrous assemblies and yarns7.11 Acknowledgement