The buttoning of the jacket is primarily determined by the button stance, a measure of how high the buttons are in relation to the natural waist. In some (now unusual) styles where the buttons are placed high, the tailor would have intended the suit to be buttoned differently from the more common lower stance. Nevertheless, some general guidelines are given here.
Double-breasted（雙排扣） suit coats are almost always kept buttoned. When there is more than one button to fasten (as in a traditional six-on-two arrangement), only the top one need be fastened; in some configurations, the wearer may elect to fasten only the bottom button, in order to present a longer line (a style popularised by Prince George, Duke of Kent).
Single-breasted（單排扣） suit coats may be either fastened or unfastened. In two-button suits（兩粒扣西服） the bottom button is traditionally left unfastened except with certain unusual cuts of jacket. Legend has it that King Edward VII started the trend of leaving the bottom button of a suit as well as waistcoat undone.
When fastening a three-button suit（三粒扣西服）, the middle button is fastened, and the top one sometimes, but the bottom is traditionally not designed to be (although in the past some jackets were cut so that it could be fastened without distorting the drape; this is not the case with current clothing). A four-button suit（四粒扣西服） is untraditional and so has no traditional guidelines on buttoning, but the middle ones at least should be fastened. Additionally, the one button suit has regained some popularity (it is also one of the classic styles of Savile Row tailoring). The button should always be fastened while standing.
With a single-breasted suit, it is proper to have the buttons unfastened whilesitting down to avoid an ugly drape. A good double-breasted suit is usually able to be left buttoned, to avoid the difficulty of constantly redoing inner buttons when standing up.
Art of Manliness Suit School:Part III - A Primer on Suit Buttons
單排扣西服扣法來自於King Edward VII，因為他太胖了，扣子扣不上，上行下效，於是開啟了西服扣子玩法新時代。
並未查到 @鄭躍成 提及的漁民說，這點存疑吧。
The tradition of men not buttoning the bottom button of waistcoats is said to be linked to Edward, who supposedly left his undone because of his large girth.
via Edward VII
@VongChunkit 提及的葬禮扣法。
There are exceptions (some people like to button all three in very formal situations, like if you"re a banker, lawyer, or going to a funeral) to the rule
via Remember the "Sometimes, Always, Never" Rule When Wearing a Suit Coat
When do you button ALL buttons? Almost never, unless attending a funeral.
via Should I button up My Suit?
是涉及到職業問題、場合問題的。
女裝西服的話。
Suit etiquette for women
Suit-wearing etiquette for women generally follows the same guidelines used by men, with a few differences and more flexibility.
via Suit (clothing)
基本按男裝走，但是可以多些靈活性。
一些討論：
Two-button suitjacket: You can either button the top button (like men) or button both buttons. You can unbutton when you sit, just like guys.
Three-button suitjacket: Button the middle button, the top two buttons or all three buttons. Now, if you are long-waisted, buttoning all of the buttons will show off your long torso. If you』re short waisted, you might want to not button that bottom button so that the jacket fits correctly.
Buttoned-jacket style tip: If you』re going to keep all your buttons buttoned all day, wear a thin belt for a little excitement.
Of course, all kinds of fashion choices are made by designers. Some jackets have no buttons. That』s fine. That type of jacket is going to be more trendy and less classic.
via Women』s Blazers Suit Jackets: To Button or Not...
這段討論對雙扣，三扣都做了說明，規則上沒男裝那麼嚴格，唯一要注意的就是style tip。最後一句話說得真好，all kinds of fashion choices are made by designers，女裝不必很嚴格。
Generally Laurel- the jacket does need to button up.
However- most of the time- the buttons on most suit coats (for men and I believe women too) aren"t buttoned.
If you think the suit coat looks OK open and your blouse is pressed and looking sharp, I would think that you will be OK.
via Help! Women"s Suit Etiquette??
Suit rules for women are nowhere near as codified as suit rules for men.
