When in doubt, use this (n-2) equation to determine how many buttons to button, where n stands for the total number of buttons on the shirt.
Taking its name from the uniform of rowers in the town of Henley-on-Thames, the henley is a versatile layering piece for virtually any time of the year. Essentially a polo/rugby shirt without a collar, the henley usually sports a placket with a few more buttons. Opt for heavier materials such as slubby cotton or wool blends to ensure that the shirt maintains its shape and, as always, keep it slim. The slimmer the shirt, the more layers you can comfortably stack on.