CHRISMON. - In diplomatics this name designates the monogrammatic formula of invocation to the Divinity that opens the protocol of a document (v.). In the late Middle Ages it also takes the name labarum (cf. C. Du Cange, s. v. in Glossarium mediae et infimae latinitatis, V, new ed., Niort 1885, p. 2).
In its original form it is represented by the so-called Constantinian monogram (v. MONOGRAMMA COSTANTINIANO E MONOGRAMMA CRISTOLOGICO): and it is to this sign that the term c. belongs in the strict sense. Over the course of the centuries, and from country to country, it nevertheless underwent various transformations, often becoming complicated with ornamental signs that totally altered its appearance: so much so that in the private acts of the 10th and 11th centuries it reveals an absolute ignorance, on the part of the scribe, not only of the origin, but also of the meaning of the c. The most common forms range from the simple cross (sometimes also decussate, and, in the Greek or Latin form, now plain, now potent, now pommelled, now anchored), to a C (initial of the Latin name Christus), for the most part intertwined with a long J (initial of Jesus). Often the c. is added to the verbal invocation; sometimes it is accompanied by a pious formula, expressed in Tironian notes. In the pontifical chancery the monogrammatic invocation was used sporadically, down to Leo IX; in the 11th century Gregory VII definitively suppressed it. The verbal one is encountered only once in papal documents (an act of Alexander II, of 18 May 1972), for which one must think that the meaning of the c. was always understood in its value.
Improperly, the name of c. is sometimes also given to the sign of the cross or a similar sign that accompanies the subscriptions of the document: in this case it has simply the value of a religious symbol and for the most part denotes the personal intervention of the subscriber in the drafting of the act, whether when there is an autograph signature, or when it takes its place.