The number of thin places in a yarn has a close relationship with the definition of its size and length. Several different definitions of thin places are introduced in this paper. The image-analysis system is then used to test the diameter of real yarns continuously. Data on the continuous diameter of ideal yarns are obtained from computational simulation. The 2D yarn-fault-analysis program designed according to a comprehensive definition of yarn faults can give over-all results of yarn faults in a mapping table in which the differences are shown in a comparison of worsted wool yarns and combed-cotton yarns with corresponding ideal yarns. Other important work is also carried out. New terminology (an index of yarn faults) is introduced in order to determine more clearly the differences between yarn faults of real yarns and those of their ideal yarns at different levels.