In her post this morning (well, morning for me, at least), fellow blogger Carolyn posed the question of why closures on womenswear are right over left, while those on menswear are left over right.There some commonly given answers to this, and her commenters, including yours truly, faithfully provided these: 1. men dressed themselves, women were dressed, making this closure easier for their usually right-handed maids or 2. It was a decency thing, this way the person helping a lady onto her horse, or holding the animal while she was mounted side-saddle couldn't peek between the buttons of her riding clothes. Those are the ones I knew. But they raise questions. Which way would you sit when riding side-saddle? How about the closures on the maid's dress? And when did this closure-rule start anyway?
I could do something to answer the last question. I own a book on 19th century fashion, but as I had expected, all the clothes in it closed the way we would expect now: left over right for men, right over left for women.
Luckily, I own another book on historical costume: the great 'Costume in detail' by Nancy Bradfield. This book is almost literally overflowing with detailed drawings of clothes from 1730 to 1930, also showing the insides and undergarments which you normally don't get to see. I can recommend it if you're a clothing, pattern and history geek like me.
What I found there was interesting.

This ladies' riding jacket, dated between 1720 and 1750, closes left over right.

So does this unusual button-fronted stomacher from 1766.

And this coat from 1828 as well.
The jacket and the coat both seem to be utilitarian garments. Obviously cut for women, but less decorative than the fashion items of their times, obviously ment for outdoor activity and inspired by menswear in their details. Of course, the normal dress of the 18th century was open-fronted with petticoats underneath and a stomacher covering the chest between the edges of the dress. There simply were no overlapping closures in fashionable ladies' clothes. The buttoned stomacher is pictured with the comment that it's a highly unusual item.
The coat is from a time when, after a few decades of empire-line dresses, the dress waist was starting to return to the position of the natural waist. Of the empire-line items, there are no clear images showing button closures, but surplice bodices are closed left over right.
After 1835, small waists and big skirts are back and this time, they are often separated. 'Dresses' are now in many cases bodices with separate skirts. And these bodices are sometimes closed at the front, with buttons. And if so, they are closed right over left, like in this 1865 example.

So, that's it then. The 'button-rule' is apperently a 19th century invention, made when buttoned clothes for women were starting to become common for every day wear. It seems that, historically, button closures entered womenswear as a menswear inspired fashion. Much like the left over right button flies on women's jeans today (other women's trousers with fly fronts usually close right over left and what's the point of a button fly for a woman?)
They were copied as they were as long as they were only used for special outdoor kit, but it seems that when they started to be seen a lot, a 'female variation' was made.
Does anyone know whether it is really in the Bible that women are not allowed to wear men's clothes? I know people believed this in the Middle Ages, and for some time after that. It was one of the reasons Joan of Arc was convicted as a witch and a heretic. That would explain the need to differentiate. (of course I'm aware that in many christian groups today, women don't wear trousers. I just don't know where biblical law meet time-honored tradition in this case)
Well, that's my twopence on the button-rule. I hope I satisfied some curiosity, but I think I mainly raised more questions. I welcome your insight in this matter!