Single Breasted Suits & Blazers
The first rule to remember is, a suit jacket should always remain buttoned when standing, however, that said, there are always exceptions to this rule.
One-button jackets should always be buttoned when standing and unfastened when seated.
Two-button jackets, the top button should remain buttoned while standing with the bottom button left undone.
Here’s an excellent example of a two-button coat with a nice natural lapel roll that comes from a canvassed lining
Three button jackets, one can either button the top two and leave the bottom unfastened, or simply button the centre button.
With a double-breasted suit, one may either fasten every button except the bottom or opt for the Military style and button all. However, with both of these rules, the jacket must remain buttoned until it is taken off, (applies whether sitting or standing). Although, some of the more traditional members of the Royal family, often utilise the Military in favour of the casual style rule.
A Waistcoat, unlike a jacket, has no buttoning rules. Period.
Side note: Unfortunately in recent years, at least since the 1960s and 1970s, as more people have grown familiar with the eccentricities of the gentry, the jacket button rule (last unfastened) was incorrectly applied to waistcoats. However, this never was or has been the case despite the confusion. A waistcoat has no buttoning rules for standing or sitting, and the bottom button may remain fastened or unfastened. However, it is common to unfasten the bottom button when seated, generally to maintain the cut and sweep of the waistcoat. This is not to say one can snarl at someone who has unbuttoned whilst standing, as this is also acceptable.
Some History and theories of the button rule
The origins of the bottom button rule, at least amongst the Lords and Ladies are merely related to Horse riding and the fact it is quite uncomfortable in some cases to fasten the lower jacket button without a specially designed riding jacket.
Researchers like to suggest that Edward VII was responsible for this ritual, and although we certainly know of the appearance of this rule around the end of the 19th century, it is very difficult to find a picture of Edward VII with a Waistcoat that is not buttoned completely.
However, whilst being officially photographed at Balmoral Castle c. 1905, the future King’s unfastened Waistcoat button was noted, therefore, one can consider this as the first time this rule was documented, and therefore one of the reasons why Edward VII was thought to have been responsible for the Jacket Button Rule.