Signum manus (sometimes also known as Chrismon) refers to the medieval practice, current from the Merovingian period until the 14th century in the Frankish Empire and its successors, of signing a document or charter with a special type of monogram or royal cypher.
Source: Signum manus – Wikipedia
Tim B.
“The signum manus in the form of a modified cross symbol first appears in charters of both Frankish Gaul and Anglo-Saxon England in the late 7th and early 8th century. Charlemagne first used his cruciform monogram, likely inspired by the earlier papal monograms, in 769, and he would continue to use it for the rest of his reign. The monogram spells KAROLVS, with the consonants K, R, L, S at the ends of the cross-arms, and the vowels A, O, V displayed in ligature at the center.[8] “