Aims and objectives
In this study, we consider the acquisition of allophonic contrast and phonetic detail in lateral consonants by second-generation Sylheti-English bilingual children in London, UK.

Design/methodology/approach
Acoustic analysis was conducted on productions of /l/ by Sylheti bilingual children, Sylheti monolingual adults and English monolingual children.

Data and analysis
Tokens of /l/ were elicited across initial, medial and final word positions from 14 bilingual Sylheti-English children, 10 monolingual English children, and 4 monolingual Sylheti adults. Acoustic measurements of F2–F1 were analysed using Bayesian linear mixed-effects modelling.

Findings and conclusions
Our results show that bilingual children produce monolingual-like positional patterns in Sylheti, producing very clear laterals in all positions. In contrast, bilinguals produce monolingual-like positional allophony in English, but they differ in phonetic detail, with bilinguals producing much clearer laterals than monolingual children across all positions.

Originality
This study is the first to examine the development of allophonic contrast and phonetic detail in both of a bilingual’s languages in a contact scenario. This provides new insights into how contact varieties adopt aspects of structure and detail from each language. We also report valuable data from Sylheti-English bilinguals, who are an understudied community.

Significance/limitations
Our study highlights the value of considering structural and detailed aspects of cross-linguistic sound systems, whereby one aspect may show monolingual-like patterns and another aspect may show distinctive patterns. We propose that the results in this study represent the development of a new sound system out of language contact, with second-generation bilingual children producing a hybrid system that combines influences from both heritage and host languages.