Sanskrit deploys extensive phonological alternations on different linguistic levels through "sandhi" rules (literally, the rules of "putting together, union, connection, alliance"), similar to the English alteration of "going to" as "gonna". The Sanskrit language accepts such alterations within it, but offers formal rules for the "sandhi" of any two words next to each other in the same sentence or linking two sentences. The external "sandhi" rules state that similar short vowels coalesce into a single long vowel, while dissimilar vowels form glides or undergo diphthongization. Among the consonants, most external "sandhi" rules recommend regressive assimilation for clarity when they are voiced. These rules ordinarily apply at compound seams and morpheme boundaries. In Vedic Sanskrit, the external "sandhi" rules are more variable than in Classical Sanskrit.