[*] 16. In compounds with prepositions the final consonant in the preposition was often assimilated to the following consonant, but usage varied considerably. There is good authority for many complete or partial assimilations; as, for ad , acc-, agg-, app-, att-, instead of adc-, adg-, etc. Before a labial consonant we find com- (comb-, comp-, comm-), but con- is the form before c, d, f, g, cons. i, q, s, t, cons. v; we find conl- or coll-, conr- or corr-; cō- in cōnectō, cōnīveō, cōnītor , cōnūbium . In usually changes to im- before p, b, m. Ob and sub may assimilate b to a following c, f, g, or p; before s and t the pronunciation of prepositions ending in b doubtless had p; surr-, summ-, occur for subr-, subm-. The inseparable amb- loses b before a consonant. Circum often loses its m before i. The s of dis becomes r before a vowel and is assimilated to a following f; sometimes this prefix appears as dī-. Instead of ex we find ef- before f (also ecf-). The d of red and sēd is generally lost before a consonant. The preposition is better left unchanged in most other cases.
[*] 16. In compounds with prepositions the final consonant in the preposition was often assimilated to the following consonant, but usage varied considerably. There is good authority for many complete or partial assimilations; as, for ad , acc-, agg-, app-, att-, instead of adc-, adg-, etc. Before a labial consonant we find com- (comb-, comp-, comm-), but con- is the form before c, d, f, g, cons. i, q, s, t, cons. v; we find conl- or coll-, conr- or corr-; cō- in cōnectō, cōnīveō, cōnītor , cōnūbium . In usually changes to im- before p, b, m. Ob and sub may assimilate b to a following c, f, g, or p; before s and t the pronunciation of prepositions ending in b doubtless had p; surr-, summ-, occur for subr-, subm-. The inseparable amb- loses b before a consonant. Circum often loses its m before i. The s of dis becomes r before a vowel and is assimilated to a following f; sometimes this prefix appears as dī-. Instead of ex we find ef- before f (also ecf-). The d of red and sēd is generally lost before a consonant. The preposition is better left unchanged in most other cases.