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Summary The mint family (Lamiaceae) is well documented as a rich source of terpene natural products. More than 200 diterpene skeletons have been reported from mints, but biosynthetic pathways are known for just a few of these.We crossreferenced chemotaxonomic data with publicly available transcriptomes to select common selfheal (Prunella vulgaris) and its highly unusual vulgarisin diterpenoids as a case study for exploring the origins of diterpene skeletal diversity in Lamiaceae. Four terpene synthases (TPS) from the TPS‐a subfamily, including two localised to the plastid, were cloned and functionally characterised. Previous examples of TPS‐a enzymes from Lamiaceae were cytosolic and reported to act on the 15‐carbon farnesyl diphosphate. Plastidial TPS‐a enzymes using the 20‐carbon geranylgeranyl diphosphate are known from other plant families, having apparently arisen independently in each family.All four new enzymes were found to be active on multiple prenyl‐diphosphate substrates with different chain lengths and stereochemistries. One of the new enzymes catalysed the cyclisation of geranylgeranyl diphosphate into 11‐hydroxy vulgarisane, the likely biosynthetic precursor of the vulgarisins.We uncovered the pathway to a rare diterpene skeleton. Our results support an emerging paradigm of substrate and compartment switching as important aspects of TPS evolution and diversification.