In Spanish dialectology, the realization of coronal fricatives is one of the most prominent features distinguishing various dialect regions. The main three realizations are the phonemic distinction between /θ/ and /s/ (distinción), the presence of only alveolar [s] (seseo), or, less commonly, the presence of only a denti-alveolar [s̟] that is similar to /θ/ (ceceo).

While an urban legend attributes the presence of the dental fricative to a Spanish king with a lisp, the various realizations of these coronal fricatives are actually a result of historical processes that date to the 15th century.

A persistent urban legend claims that the prevalence of the sound /θ/ in Spanish can be traced to a Spanish king who spoke with a lisp, whose pronunciation spread via prestige borrowing to the rest of the population. This myth has been discredited by scholars. Lundeberg (1947) traces the origins of the legend to a chronicle of Pero López de Ayala which says that Peter of Castile "lisped a little" ("ceceaba un poco"). However, Peter reigned in the 14th century and the sound /θ/ began to develop in the 16th century (see below). Moreover, a true lisp would not give rise to the systematic distinction between /s/ and /θ/ that characterizes standard Peninsular pronunciation. For example, a lisp would lead one to pronounce siento ('I feel') and ciento ('hundred') the same (as [ˈθjento]) whereas in standard Peninsular Spanish they are pronounced [ˈsjento] and [ˈθjento] respectively.

The misnomer "Castilian lisp" is used occasionally to refer to the presence of [θ] in Peninsular pronunciation (in both distinción and ceceo varieties).

In the 15th century, Spanish had developed a large number of sibilant phonemes: seven by some accounts, eight by others (depending on whether /d͡ʒ/ and /ʒ/ are considered contrasting), more than any current dialect. During the 16th and early 17th centuries these phonemes merged differently as they evolved into those of the different modern dialects. There were four pairs of voiceless versus voiced sibilants: dental/alveolar affricates /t͡s/ vs. /d͡z/ (spelled ⟨c⟩ or ⟨ç⟩ vs. ⟨z⟩); dental/alveolar fricatives /s/ (spelled ⟨ss⟩ when intervocalic, ⟨s⟩ otherwise) vs. /z/ (intervocalic only, spelled ⟨s⟩); postalveolar affricates /t͡ʃ/ (spelled ⟨ch⟩) vs. /d͡ʒ/; and postalveolar fricatives /ʃ/ (spelled ⟨x⟩) vs. /ʒ/. Both /d͡ʒ/ and /ʒ/ were spelled ⟨g⟩ before ⟨e⟩ or ⟨i⟩, and ⟨j⟩ elsewhere. It is likely that /d͡ʒ/ deaffricated and merged with /ʒ/ before the year 1500. The main difference between the prestige dialect of north central Spain and dialects to the south (such as Andalusian Spanish) was that, in the north, the dental/alveolar continuants were more retracted than the affricates (the former pair can be represented as /s̺/ and /z̺/ and the latter as /t͡s̪/ and /d͡z̪/),^{[clarification needed]} keeping their phonemic distinction, while in the south they were homorganic. The first step away from that system was the deaffrication of /d͡z̪/ in the first quarter of the 16th century. Because of a differing place of articulation, this still contrasted with /z̺/ in the prestige dialect of north central Spain, though it was a complete merger for southern dialects.

The second step was the devoicing of voiced sibilants. In the north, /z̺/ and /ʒ/ were lost, but /z̪/ remained contrastive with its new pronunciation /s̪/, because there had been no voiceless /s̪/ previously. This sound contrasted with two acoustically similar sounds: dentoalveolar /t͡s̪/ and apicoalveolar /s̺/. By 1600, /t͡s̪/, too, had deaffricated and merged with the earlier /s̪/ that had already developed from /z̪/. Subsequent changes to the sound system of Spanish retained the contrasts while enhancing the segments by increasing articulatory distance amongst their rather subtle acoustic contrasts, an appropriate step due to the high productivity of these phonemes in differentiating frequently used minimal pairs. The dentoalveolar one was moved "forward" to interdental /θ̟/, losing its former sibilance in the process (which increased its acoustic distance to the remaining sibilant ⟨s⟩), and the prepalatal one was moved "backward" to velar /x/, also losing its former sibilance, all in all resulting in the three-way distinction of modern Standard Peninsular pronunciation:

In the south, the devoicing process and deaffrication of /t͡s/ gave rise to new fricatives that were indistinguishable from the existing ones. The process of increasing articulatory distance still applied, however, and /ʃ/ retracted to /x/ in the south just as it did in the north. In a number of ceceo areas (particularly the southernmost provinces like Cádiz) /s/ developed into a non-sibilant apico-dental [θ̺], perceptually similar to the interdental /θ̟/ used by Standard Peninsular speakers for orthographic ⟨c⟩/⟨z⟩. In seseo areas (particularly in the westernmost provinces like Seville and Huelva), the resulting phoneme developed a predorsal alveolar realization [s̻] (like English ⟨s⟩), perceptually similar to the apicoalveolar [s̺] used by Standard Peninsular speakers for orthographic ⟨s⟩. This seseo variety was the pronunciation that most impacted Latin America, as many emigrants to the Americas were from Andalusian and Canarian ports. In addition, several generations of Spanish speakers had lived and grown in the Americas before /θ/ appeared in Castilian.

The development of the sibilants in Ladino (which split off from Castilian and other Peninsular varieties in the 15th century) was more conservative, resulting in a system closer to that of Portuguese.