Most people already know the etiquette regarding single breasted waistcoats, that being that the bottom button should be left undone. There is no point going in to the reasons why this may be – there is no hard evidence for corroborate any of the myths as to its origin – the bottom line is, you should leave the bottom button of your single breasted (day) waistcoat undone at all times. However, what to do with a double breasted waistcoat is less clear. You may see some bottom buttons left undone on double breasted waistcoats, but there doesn’t seem to be any historical precedent for this, so it most likely an over-generalisation of the single breasted rule. On balance, it is my opinion that you should fasten all buttons if you are wearing a double breasted waistcoat.
Sometimes a waistcoat will feature an additional buttonhole for the purpose of attaching a watch chain. On single breasted waistcoats, this is generally found in line with the front buttons, usually in a vertical orientation. On double breasted waistcoats they can appear either in the dead centre or hidden underneath the lapel. These are fairly rare and are generally found on vintage models only, though some modern makers, such as Hackett, still include them. They are by no means essential, however, and if you find your waistcoat doesn’t have one, don’t worry: there are plenty of other ways to attach a watch chain if you choose to wear one.