The third mode of utterance is by means of the letter (_epistola_) or rescript (_rescriptum_).[1816] These letters contained instructions either on administrative or on judicial matters. In their first character they might be addressed either to individual officials subordinate to the Emperor or to the provincial diet,[1817] the scope of their application depending on the Emperor’s discretion at the time of the issue, and on the interpretation of the rescript after his death. In matters of justice, whether addressed to the judge or to the litigant, they might settle doubtful points of law or extend a principle to new cases. The power of interpretation is at least as great in the rescript as in the edict; but the rescript was the more powerful vehicle for law-making. It kept the Princeps in constant touch with the provincial world, and was the chief mode in which the uniformity of its administration and its law was moulded. The rescripts also had, on account of the precision and permanence of their form, a more unquestioned validity, as perpetual enactments, than either the edict or the decree. When the _acta_ of an emperor are referred to, it is chiefly these, together with the charters or privileges (_leges datae_, _beneficia_) that he may have conferred on states, that are intended. The rescripts might be elicited either by the _consultatio_ of a doubtful official who was subordinated, either as an administrator or as a judge, to the Emperor, or they might be written in answer to the petition (_libellus_, _supplicatio_) of one of the parties to a suit. In the latter case they were often a convenient substitute for the personal appearance of the appellant in the Emperor’s court.