because monolingual Spanish-learning children learn to produce the continuant contrast between and before they do the lead voicing contrast between and . (In comparison, English-learning children are able to produce adult-like voicing contrasts for these stops well before age three.) The allophonic distribution of and produced in adult speech is not learned until after age two and not fully mastered even at age four. The alveolar trill is one of the most difficult sounds to be produced in Spanish and as a result is acquired later in development. Research suggests that the alveolar trill is acquired and developed between the ages of three and six years. Some children acquire an adult-like trill within this period and some fail to properly acquire the trill. The attempted trill sound of the poor trillers is often perceived as a series of taps owing to hyperactive tongue movement during production. The trill is also very difficult for those learning Spanish as a second language, sometimes taking over a year to produce properly. Codas One research study found that children acquire medial codas before final codas, and stressed codas before unstressed codas. Since medial codas are often stressed and must undergo place assimilation, greater importance is accorded to their acquisition. Liquid and nasal codas occur word-medially and at the ends of frequently used function words, so they are often acquired first. Prosody Research suggests that children overgeneralize stress rules when they are reproducing novel Spanish words and that they have a tendency to stress the penultimate syllables of antepenultimately stressed words, to avoid a violation of nonverb stress rules that they have acquired. Many of the most frequent words heard by children have irregular stress patterns or are verbs, which violate nonverb stress rules. This complicates stress rules until ages three to four, when stress acquisition is essentially complete, and children begin to apply these rules to novel irregular situations. Dialectal variation Some features, such as the pronunciation of voiceless stops , have no dialectal variation. However, there are numerous other features of pronunciation that differ from dialect to dialect. Yeísmo One notable dialectal feature is the merging of the voiced palatal approximant (as in ayer) with the palatal lateral approximant (as in calle) into one phoneme (yeísmo), with losing its laterality. While the distinction between these two sounds has traditionally been a feature of Castilian Spanish, this merger has spread throughout most of Spain