Black Wrist Ribbons

We've all seen the ubiquitous black silk ribbon around the necks of women and girls in portraits. But what of these wide black ribbons worn around both wrists? Who wore them, under what circumstances were they worn, and most importantly, how did they fasten?!

                                  Mrs. Abingdon, by Sir Joshua Reynolds. 1771

                                           
                                Mrs. Stanhope, by Sir Joshua Reynolds. 1774

Mrs. Clark Grayton, by John Singleton Copley, 1779