INDIZIONE. – A chronological computation based on a fifteen-year cycle, whose years are numbered successively from 1 to 15 and designated respectively as i. I, II, III, etc. After i. XV, the numbering begins again from 1, always limiting itself to mentioning the year of each cycle and disregarding any indication relating to the series of the cycles themselves. The i. is therefore not in itself a sufficient chronological indication, but must be related to other dates (the Christian era, years of reign or pontificate, etc.); it nevertheless constitutes an element of verification and is of great importance in diplomatic criticism.
Its use goes back, in Egypt, at least to A.D. 29 (cf. A. Calderini, Papyrri, Milano [1944], p. 35 ff.); it seems that, in that region, the indictional period corresponded to the span of time between one cadastral survey and the next; the computation was presumably first introduced in documents relating to taxation and from there extended to other acts: having entered liturgical practice, its use became general.
The indictional year, like the year of the Christian era, did not everywhere begin on the same day, so that in checking one must take into account the possibility that the indictional figure may appear increased or decreased by one unit, depending on the style followed in that period and in that region. The following were in use: a) the Greek, or Constantinopolitan, i., beginning on 1 Sept., and thus preceding the year of the circumcision by 4 months: it was characteristic of the court of Byzantium, used preferably in papal documents from Pelagius II to Vittore III, employed in royal documents until 832, and very widespread in public and private acts of southern Italy; b) the Bedan i., so called by Grotefend because it is first attested in s. Beda, also improperly called Caesarean or Constantinian, beginning on 24 Sept., and thus preceding the year of the circumcision by 3 months and 6 days: it is found in the imperial chancery until 850, variously in Italy, especially central Italy, and is widely attested in the episcopal documents of Germany until the middle of the 14th century. In the territory of Genova it is encountered with a delay of one year, that is, with one unit less than in the common Bedan computation; c) the Roman or pontifical i., beginning on 25 Dec., and subsequently on 1 Jan., thus coinciding first with the year of the nativity and later with that of the circumcision; it is attested in Rome at the end of the 10th century; from 1088 its use begins in papal letters, and in the 13th century it is also found in the German chancery; it then becomes generally used and today is the only one surviving in the liturgy; d) the Sienese i., beginning on 8 Sept., and preceding the year of the circumcision by 3 months and 22 days; limited in use to the territory of Siena, it is always found in synchrony with the year ab incarnatione in the Florentine style.
Starting from the earliest evidence for the use of the i., it was calculated hypothetically that the year 1
A.D. corresponded to i. IV, that is, that the beginning of the Christian era was three years later than the beginning of an indictional cycle: «Quarta indictione secundum Dionysium natus est Dominus» (Beda, De temporum ratione, 49: PL 90, 496). From this observation arose the formulation of the rule for calculating the i., which is obtained by adding 3 units to the figure of the year of the Christian era and dividing the sum by 15: the remainder gives the i. (a remainder of zero corresponds to i. xv), while the quotient, which has no importance for chronological calculation, represents the hypothetical number of indictional cycles from the beginning of the common era to the date for which the calculation is being made.