Eleanor Horner

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16/09/2026

The Latin demonstrative pronouns appear to be able to be supplemented with the suffix -dem: ea + dem > eadem, etc.. According to this, you would expect the m. sg. to be isdem, not īdem. The unexpected form can be explained by a reanalysis, analogy and a regular sound change.

The productive suffix was in fact not -dem, but -em. It could be added to is and id (PIE *(h­₁)is and *(h₁)id) to make *isem and idem. The reanalysis here occurred in the neuter: id-em was reinterpreted as i-dem.

Then the rest of the paradigm was adapted in analogy therewith, including *is-em > is-dem. Interestingly, eōdem and eādem (cf. eō and eā) were not formed through this analogy; instead, they were among the forms that supported it, because eō and eā originally ended with a -d.

Then came the sound change: Vsd > V̄d. Accordingly, isdem became īdem. Although this happened in the third to fourth century BC (i.e. before the written record), ISDEM is in fact attested as an archaism in a source from 45 AD.

The words sīdere and nīdus are also examples of this sound change. Both come from the PIE root *sed („to sit“). This verb forms the present tense thematically, in the form Ci-CC-e/o-: *si-sd-e/o-. With the prefix *ni- (“down”¹), you can form *ni-sd-os (“a place to sit down”¹). Both words (*sisdo- and *nisdos) display the PIE /isd/ that would become Latin /īd/.

Notes

¹ „nieder“; „Ort zum Niedersitzen“

Sources

Meiser, G. 1998. Historische Laut- und Formenlehre der lateinischen Sprache.

Sommer, F. 1914 & 1977. Handbuch der Lateinischen Laut- und Formenlehre (the „second and third“ edition from 1914 and the fourth edition from 1977).

Weiss, M. 2020. Outline of the Historical and Comparative Grammar of Latin (second edition).