ABSTRACT In recent years, social aspects of teaching and learning have gained increased recognition, prompting researchers to focus on conversations and language in mathematics classrooms. While prior studies have highlighted the importance of conversations in primary school teaching, a gap remains in understanding how they influence mathematics teaching and learning, especially among younger children (Muhonen et al. 2016; Patterson 2018). To address this gap, this study explores the implementation of ground rules for talk in two first-grade classrooms in Norway (ages 5–6 years). This educational design research study investigated the impact of conversation rules on enhancing mathematical reasoning and promoting productive communication in primary school. Findings indicate that implementing ground rules for talk can foster productivity by encouraging pupils to inquire about others’ reasoning and share their perspectives to foster a respectful discourse. However, it also emphasises the importance of teachers being mindful of their language to foster reasoning effectively.