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Measured by the neck in one-centimetre increments and two-centimetre increments in shirts over 50cm neck.
A fractional fit gives you a precise fit when wearing a tie. If you do not intend to wear a tie and will leave the top button undone then the neck size is not as crucial for your purchase.
Regarding Van Heusen and Ganton shirts, some of their shirt styles come with a choice of sleeve lengths which adds to your fractional fit.
The general description of a small, medium, large, etc has size increments of 2cm in the neck and 5cm in the chest. Shirts that have this description only, are shirts not intended to do the top button-up such as casual shirts.
These days shirts can come in three variables eg classic fit, tailored fit, and slim fit. These body shapes are the amount of decreasing measurement between the chest and the waist of a shirt. Eg the slim fit shirt will have the chest with the smallest waist.