A more thorough explanation:

Diplomatics is the science of deciphering and authenticating ancient writings. It involves studying old documents like letters, wills, deeds of conveyance, public treaties, and other similar instruments to verify facts and establish public and private rights. The principles of diplomatics were largely developed by the Benedictine Dom Mabillon in his 1681 work entitled De re diplomatica.

One example of diplomatics is the study of ancient diplomas, which were written on two leaves or double tablets. The Romans used the term more specifically for letters of license to use public conveyances provided at different stations and generally for public grants. With the revival of literature, the importance of such documents in verifying facts and establishing public and private rights led to their being brought together from historical works and monastic registers in which they had been copied, or in rarer instances, from public and ecclesiastical archives where the originals were still preserved.

Another example is the use of diplomatics to distinguish genuine documents from forgeries. Treatises were written on the subject of these diplomas to establish principles for distinguishing the genuine from the forged. Disputants defended or condemned them, and doubts of the so-called originals arose.

These examples illustrate how diplomatics is used to study and authenticate ancient writings, including diplomas, letters, wills, and other similar instruments. It is an important field of study for verifying historical facts and establishing public and private rights.