The way that individuals use function words in a conversation-reflecting how they say things, rather than what they say-is called their individual language style. The dyadic coordination of language styles, called language style matching (LSM), is central to the development of social relationships in conversations. Despite a growing body of research on LSM, conceptual and methodological approaches are inconsistent between scholars. After giving a conceptual overview of LSM, we derive the properties desirable for analyses of LSM in interaction (e.g., reciprocity, consistency, and frequency sensitivity). Building on these properties, the existing three methodological approaches to LSM are reviewed. Since none of the existing metrics fulfills all the desired properties, we introduce a new metric to assess LSM in dyadic interaction, capturing reciprocal adaption throughout the dynamic process of a conversation. Hence, the new metric is called reciprocal LSM (rLSM). To empirically establish the conceptual underpinnings of rLSM, the metric is compared to the LSM metric most commonly used in psychological research. Both metrics are applied to a set of N = 77 transcribed real-life dyadic conversations, analyzed with the Linguistic Inquiry and Word Count software. The results indicate that rLSM is a better estimate of LSM than is the old metric and that there is high conceptual similarity between the two metrics. Implications for existing research and directions for future research are discussed. To facilitate the standardization and comparability of research, guidelines are provided for authors on the use of the new and existing metrics.