My sister and I were given middle names. When I got into genealogy, I could find no middle names for my paternal grandmother or her daughter, my Aunt Barb. It turns out that side of the family did not give girls middle names because girls, of course, would get married and use their maiden name as their middle name.
So my grandmother is Ruth Bressler Claycomb – no need to jettison a middle name. Her sisters, Kate, Maud, and Dorothy also had no middle names.
Her daughter became Barbara Claycomb Davis.
Taking one’s maiden name as a middle name is very common in our family. My mother was born Harriet Lorraine Duncan, and took Duncan as her middle name. Off hand, I can’t think of any women who kept their original middle name when they married.
Of course, underlying this is the assumption that all young women will get married or at least aspire to marry. And that they will take their husband’s name. Things have changed – women keep their original names or hyphenate them or go a more traditional route. It just makes more possibilities for genealogists to sort out.