Spanish is a language characterized on the phonetic level by a rich variation in consonantism, especially in the syllable-final position (both word-inner and word-final), whereas vocalism shows a more fixed character and a less relevant variation. Thus, it is not strange that the majority of variationist studies have focused on consonantism. Investigations addressing prosodic variation are fewer and more recent and will not be broached here because of space limitations.
In the field of consonant variation, studies focusing on the weakening effect on certain elements of the coda stand out. The most relevant among these phenomena is the one which affects /s/, and this is so for various reasons: (a) because has been considered to be one of the isoglosses that divide the two great norms of current Spanish (Castilian and Atlantic); (b) because of its geographical extent; and (c) because it has led to theoretical approaches about the possible impact on number (singular/plural) and grammatical person (second/third-person singular) differentiation that implies the frequent presence of word-final -s. Additionally, variation which affects the liquid consonants (/l/ and /r/), leading to processes of both weakening and change in these two types of consonants, has been studied quite extensively (at least in Andalusia and the Canary Islands). The weakening process affecting the final nasal consonant, with velarization as a first step and potential elision as a second one, has been less frequently studied in Spain (much less than in the case of Caribbean Spanish, for example). In the field of syllabic tension, there is another phenomenon that has often been studied due to its geographic extent: the weakening of intervocalic /d/, which yields very different data depending on dialectal variety (evidence of the greater or lesser degree of progress in that weakening process). Sociolinguistic analyses also focus on the increasing expansion of yeísmo, a phenomenon usually conditioned by age as an explanatory factor in this advanced stage of the process. In Andalusian Spanish, the alternation between different pronunciations of the phonemes /s/ and /θ/ (mainly distinction, seseo and ceceo), the defricativization of /tʃ/ and the alternation between the realizations of /x/ as [x] and [h] have also been analyzed.
In the case of vowels, as has been said before, it should be pointed out that cases of sociolinguistic variation in Spanish are not as numerous or as relevant; therefore, they have been less appealing to researchers. Among the main phenomena, we shall discuss the vocalic metaphony registered in Cantabria and Asturias.