"Apoorva Raagangal" explores the concept of relationships between couples with a large disparity in their ages, which challenged Indian social mores. Although it is based on a "Baital Pachisi" riddle, it has frequently been compared to the American film "40 Carats" (1973), which tells the story of a widow who falls in love with a much younger man. In "Apoorva Raagangal", Ranjani often poses the riddle "Ennudaya appa yaarukku maamanaro, avarudaya marumagalin appa en maganukku maamanar. Appa avarukkum enakkum enna uravu?" (My father is father-in-law to someone; that person's daughter-in-law's father is my son's father-in-law. What is the relationship between him and me?), summing up the film's theme. While the source riddle in "Baital Pachisi" does not have an answer, G. Dhananjayan believes the answer to the film's riddle is, "for a husband, the relation is his wife and for the wife, the relation is her husband".