Fichus were worn throughout the century, in order to cover the low décolleté, more for modesty than for warmth. The right-angle point was atthe centre back, the flaps crossed over the breast and tied at the small of the back. Smaller fichus were tucked into the bodice or held with a clasp in front.
As the nobility of the 18th century was quite open-hearted, it was mostly servants and elderly ladies who wore them. The reason is probably that they often inherited their mistresses' dresses (or strove to imitate the upperclass) while not sharing their liberty. In the case of elderly ladies, it is probably clear...
During the 1770s and 80s, as traits of bourgeois fashion entered that of the upper class, fichus "trickled up" and became - in a more or less revealing style - standard.
the Turnip Girl's fichu, 1740s
an elderly lady's fichu, 1770s
large fichu, 1780s