Gratian, De legibus, Dictum ante c.1 Distinctio 1
Humanum genus duobus regitur, naturali uidelicet iure et moribus. Ius naturae est, quod in lege et euangelio continetur, quo quisque iubetur alii facere, quod sibi uult fieri, et prohibetur alii inferre, quod sibi nolit fieri. Unde Christus in euangelio: "Omnia quecunque uultis ut faciant uobis homines, et uos eadem facite illis. Haec est enim lex et prophetae (Matthew 7:12 and Luke 6:31)."

The Human Race is ruled by two things:  namely, natural law and long standing custom (mos).  Natural law is what is contained in the law and the Gospel.  By it, each person is commanded to do to others what he wants done to himself and is prohibited from inflicting on others what he does not want done to himself.

Roman law sources

Equitas in Gratian

Cologne, Dombibl. 127, fol. 9r

When Pope Innocent II invested Emperor Lothair III with Apulia in 1137 he used a lance with a banner, but the pope held the upper part of the lance.  Hubert Houben, Roger II of Sicily: A Ruler Between East and West (Cambridge Medieval Textbooks; Cambridge: 2002) 69