E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
If a lady or gentleman is willing to marry, but not engaged, a ring should be worn on the index finger of the left hand; if engaged, on the second finger; if married, on the third finger; but if either has no desire to marry, on the little finger. (Mme. C. de la Tour.)
A ring worn on the forefinger indicates a haughty, bold, and overbearing spirit; on the long finger, prudence, dignity, and discretion; on the marriage finger, love and affection; on the little finger, a masterful spirit.
Ring given in marriage, because it was anciently used as a seal, by which orders were signed (Gen. xxxviii. 18; Esther iii. 1012); and the delivery of a ring was a sign that the giver endowed the person who received it with all the power he himself possessed (Gen. xli. 42). The woman who had the ring could issue commands as her husband, and was in every respect his representative.
In the Roman espousals, the man gave the woman a ring by way of pledge, and the woman put it on the third finger of her left hand, because it was believed that a nerve ran from that finger to the heart.Macrobius: Sat. vii. 15.