When you read about clothing in the 20s and 30s, women's clothing is alternately referred to as dresses or frocks, and sometimes gowns. Now, I know what a gown is, but I did sometimes wonder just what is the difference between a dress and a frock. Turns out there really is a difference.
According to Mary Brooks Pickens' booklet "Dresses, Part I," published by the Women's Institute of Domestic Arts and Sciences (multiple printings, starting in 2016, but the one I have is from 1921),
"it might be said that a dress is an outer garment for women and children, the term applying to simple or complicated constructions, whether made in one piece or of a separate waist and a separate skirt, in harmonizing materials. A frock in the modern acceptance of the term is an outer garment for children and young women that is smart as to combination of style features and in harmonious keeping with prevailing fashions. The term gown is used in referring to elaborate court dress and outer garments for ceremonial or ecclesiastical wear, such garments being made in long, flowing lines and of elegant fabrics, elegance being the paramount thought. The term gown refers also to an outer garment for women that is either close or loose fitting, rich in fabric, and beautiful in line, the term being particularly applicable to all dresses with trains."
"A dress may be made of gingham, muslin, taffeta or chiffon, that is, of cheap or expensive material, and yet be properly called a dress. In speaking of such a garment for the mature woman, it would seem that the term dress is most befitting; but in referring to the young miss or the young matron, especially if it is attractive, bespeaks a correct assemblage of style features, and carries a suggestion of the prevailing mode, it may be correctly called a frock, because the very smartness of the name gives a clear mind picture of the actual garment. The word gown, however suggests drapery, long flowing lines and stateliness; it would indeed be very inappropriate to refer to a garment developed in dignified line and of exquisite fabric by any other word except gown."
To be sure, even Ms. Brooks Pickens said that the words were defined in different ways by different people during this time, so it's no wonder that we are confused now. Gowns are gowns, especially if they have a train, so I don't think that's very confusing. From her definitions, it sounds like a frock then would be what we call trendy now, and would only be seen on girls of a certain age. If you were "mature", you would never wear a frock -- it would always be a dress.
I guess I'm relegated to dresses, but at least I know what a frock is now.