Christian Late Antiquity: Transcription
Section 3: Abbreviations: Nomina Sacra
The nomina sacra are a small set of standard abbreviations for names of the deity. These are abbreviations by contraction. The nomina sacra usually have a horizontal line over the whole abbreviation to mark it as such.
DS stands for deus. The final S changes when the word is inflected, so, for example DM = deum, DO = deo, and DI = dei.
DNS, on the other hand, stands for dominus. It, too, changes its last letter according to the requirements of inflection (DNI, DNO, DNM).
Be especially careful not to confuse DS and DNS. It may help to remember: DS = two letters standing for a two-syllable word; DNS = three letters standing for a three-syllable word.
IHS and XPS are nomina sacra for Jesus's name, with the added complication that they are mixed Greek abbreviations used to represent the Latin forms of the names. The IH in IHS are iota and eta. (Sometimes the S is written C, to represent the Greek sigma.) In XPS, X is chi, normally spelled CH in Latin, and P is the Greek letter rho. Despite the Greek letters, you should expand these abbreviations in Latin, as follows:
IHS = ie(su)s XPS = chr(istu)s
Note that you do not put the e in iesus in parentheses when you expand, because it is present in the abbreviation's H (eta), and you do not put the h in christus in parentheses because ch together are represented by X in the abbreviation.
(Note: You do sometimes find medieval scribes spelling Jesus "ihesus", which probably arose from a misunderstanding of the IHS abbreviation. If your scribe consistently spells out ihesus with the h, then you may expand that scribe's IHS abbreviation with an h in it, too, but not otherwise!)
As with DS and DNS, IHS and XPS change their last letter according to the inflection of the word. Thus IHU is used for iesu (the form for all cases of iesus other than nominative), and we find XPI, XPO, XPM as needed.
The other two most common nomina sacra are those for the Holy Spirit: SPS SCS = spiritus sanctus. Both abbreviations also change their last letter for inflection, and you will of course find them used separately as well as together.
When you have digested all this, click through to try some practice exercises in expanding abbreviations.