CHRISMON. — In diplomatics this term designates the monogrammatic invocation formula to the Divinity that opens the protocol of a document (v.). In the late Middle Ages it also takes the name of 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. properly belongs in the strict sense. Over the centuries, however, and from country to country, it underwent various transformations, often becoming complicated with ornamental signs that completely 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 its origin but also of its meaning.
The most common forms range from the simple cross (sometimes also decussate, and in the Greek or Latin form, now plain, now potent, now pommé, now anchored) to a C (the initial of the Latin name Christus), most often intertwined with a long J (the 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 papal chancery the monogrammatic invocation was used sporadically until Leo IX; in the 11th century Gregory VII suppressed it definitively. The verbal invocation occurs only once in papal documents (an act of Alexander II, dated 18 May 1072), for which one must assume that the meaning of the c. was always understood in its proper sense.
Improperly, the term c. is sometimes used to denote the sign of the cross or similar sign that accompanies subscriptions to a document: in this case it simply has the value of a religious symbol and usually indicates the personal intervention of the subscriber in the act, whether there is an autograph signature or it takes its place.