His subject is a "strange image" among the llth century drawings of the Winchester school, which he calls a quinity. But what is a quinity? Let me explain it in his own words: "It is a Quaternity of God the Father, the Son, St. Mary, and the Holy Ghost; or, if we add the Infant on the lap of the Virgin, we face the seemingly unique representation of what logically must be called a Quinity".
Source: wiktionary