In this way arose a suffix -sus ([906], [912]) for the past participle of verbs ending in a dental, and this spread to other verbs ([912]): as mānsus, stayed, from maneō ([1000]), pulsus, pushed, from pellō ([932]). The regular participles of these two verbs still appear in the derivative verbs mantāre and pultāre, which presuppose the past participles *mantus and *pultus ([371]). If the double tt was followed by r it changed to st: as, assestrīx from *assettrīx, while *assettor changed to assessor.
[160]. But wherever the combination tt arose in historical times it remained unchanged: as, attineō; cette, syncopated for cé-d(i)te, i.e. the particle ce ([93, 3]) which is here proclitic, and the imperative date, give.
[161]. Initial dv (dṷ) changed to b, unless the v (ṷ) was converted into the corresponding vowel: as, bis, twice, for *dṷis (cf. duo); bidēns for *dṷidens, by the side of old Latin duidēns with vocalic u; bonus, good, for dṷonus, by the side of trisyllabic duonus; bellum, war, for *dṷellum, by the side of duellum with vocalic u; bēs, two thirds, for *dṷēs ([2427]). Cicero (O. 153) notes that the change of duellum to bellum affected even the proper name Duellius (name of the admiral who won the naval victory over the Carthaginians in 260 B.C.) which was changed to Bellius. Plautus always scans dṷellum disyllabic with synizesis ([2503]).
[CHANGES OF CONSONANT GROUPS.]
[162]. Many groups of consonants undergo changes in order to facilitate their pronunciation in rapid speech. These changes involve (a.) Assimilation of consonants; (b.) the development of consonantal glides; (c.) the loss of one member of the group; and (d.) the development of a vowel between the consonants.
[163]. Of two successive consonants belonging to different syllables ([175]), the first is, as a rule, assimilated to the second (regressive assimilation), rarely the second to the first (progressive assimilation). A consonant may be assimilated, either entirely or partially, to another consonant.
Assimilation is very common in prepositions prefixed to a verb.
[164]. Partial assimilation. (1.) A voiced mute before an unvoiced consonant became unvoiced: as, rēx, king, for *rēgs (cf. rēgis); rēxī, I guided, for *rēgsī (cf. regō); rēctus, guided, for *rēgtus; scrīpsī, I wrote, for *scrībsī (cf. scribō); scrīptus, written, for *scribtus; trāxī, I dragged, for *trāghsī; tractus, dragged, for *traghtus ([152]). The spelling did not always conform to this pronunciation: as, urbs, city, pronounced urps ([54]) but spelled with b by analogy to the oblique cases urbis, urbem, etc.; obtineō, I get, pronounced optineō.
([2.]) An unvoiced mute before a voiced consonant became voiced. The prepositions ob, ab, sub, for *op, *ap, *sup, owe their final b to their frequent position before voiced mutes: as, obdūcō, abdīcō, sub dīvō. The forms *op (still preserved in op-eriō, I close, [1019]) *ap (preserved in ap-erio, I open, [1019]; cf. Greek ἀπό) and *sup (preserved in the adjective supīnus, supine) were then crowded out by ob, ab, and sub.