Worn underneath or over the bodice, a fichu was little more than a kerchief, often pinned with a brooch. Much more popular for day use, fichus were worn by the more conservative ladies for evening events as well.
They ranged from the simplest square cloth to exquisite triangular lace creations. 
Vangie, in The Viscount’s Vow crocheted her own when the bodice of her borrowed gown was too low for her liking.
I’m wondering though, if one didn’t pin them, did the darn things keep coming untucked? I can’t imagine a proper young woman stuffing the frilly garment back inside her cleavage in full view of all.
Resources
http://en.wikipedia.org/wiki/Fichu
http://oregonregency.blogspot.com/2011/11/fabulous-fichu-regency-accessory.html
http://zipzipinkspot.blogspot.com/2009/07/regency-fichus-more-than-just-squares.html
http://www.songsmyth.com/daydresses.html
