dʰeǵʰ-
“to burn, to kindle”burn, be warm
Root for burning, yielding Latin fovere (to warm), Lithuanian degti, Old Irish daig (flame).
Discussion
The PIE root *dʰeǵʰ- (to burn, to kindle) produced reflexes in most major branches, though the semantic development varies from literal burning to warmth, day, and season — a chain of meaning shifts that illuminates how PIE speakers conceptualised time and temperature.
The most prominent descendant is Latin diēs (day), which continues the zero-grade *dʰiǵʰ- via an intermediate "burning, heat of the day" stage. The semantic path from "burn" to "day" is not obvious in English, but it becomes clear when you consider that "day" was conceived as the hot, bright period — the burning time — in contrast to the cold darkness of night. Latin diēs generated an enormous derivative vocabulary: diary, dial, dismal (originally dies mali, "evil days"), diurnal, journal (from diurnalis via French), journey (originally a day's travel), adjourn, sojourn, and meridian (medius + dies, midday).
The connection extends further through the deified form *Dyēws (sky father, literally "the shining one"), which gave Latin Jupiter (from *Dyēw-ph₂tēr, "sky father"), Greek Zeus (genitive Diós), and Sanskrit Dyáuṣ pitā. This is one of the most celebrated cognate sets in comparative linguistics — the supreme deity of three major branches sharing an identical name from the same root.
In the Germanic branch, the o-grade *dʰoǵʰ- produced the concept of warmth and baking: Old English dag (day) continues the temporal sense, while the root's presence in words related to heat and transformation survives in dough (the thing that is baked, from the burning/heating sense) and the archaic dialectal dag (dew, morning moisture — the moisture of the burning-time).
Greek preserved the root in multiple derivatives: the suffix -θεσις (-thesis, from a different root) is sometimes confused with reflexes of *dʰeǵʰ-, but the genuine Greek descendants include the element in θέρος (théros, summer, the burning season) — though this may belong to *gʷʰer- (warm) instead, and the attribution is debated.
The Balto-Slavic reflexes include Lithuanian degti (to burn) and Russian žečʹ (to burn), both transparently continuing the original "burn" meaning without the semantic shift to "day" found in the western branches. The preservation of the literal sense in the east, alongside the metaphorical extension in the west, allows reconstruction of the semantic development with unusual confidence.
The root's phonological shape — initial *dʰ (voiced aspirate), medial *e (e-grade), final *ǵʰ (palatalised voiced aspirate) — makes it a doubly aspirate root, a class that shows distinctive treatment in Greek (Grassmann's Law: one aspirate loses its aspiration, giving τ for the initial) and Sanskrit (the same dissimilation). This phonological constraint is itself evidence for the reconstruction: the regular operation of Grassmann's Law across Greek and Sanskrit reflexes confirms that both aspirates were present in the proto-form.
Notes
Pokorny 240-241. Widely attested fire root.