Some conceptual metaphors common in spoken languages are infelicitous in sign languages. The explanation suggested in this article is that the iconicity of these signs clashes with the shifts in meaning that take place in these metaphorical extensions. Both iconicity and metaphors are built on mappings of two domains: form and meaning in iconicity, source domain and target domain in metaphors. Iconic signs that undergo metaphoric extension are therefore subject to both mappings (Taub 2001). When the two mappings do not preserve the same structural correspondence, the metaphorical extension is blocked. This restriction is formulated as the double-mapping constraint , which requires multiple mappings to be structure-preserving. The effects of this constraint go beyond explaining possible and impossible metaphors in sign languages. Because of the central role of metaphors in various linguistic processes, constraints on their occurrence may affect other linguistic structures and processes that are built on these metaphors in both sign and spoken languages.