Abstract—Traditionally, metamodeling is an upfront activity performed by experts for defining modeling languages. Modeling tools then typically restrict modelers to using only constructs de-fined in the metamodel. This is inappropriate when users want to sketch graphical models without any restrictions and only later assign meanings to the sketched elements. Upfront metamodeling also complicates the creation of domain-specific languages, as it requires experts with both domain and metamodeling expertise. In this paper we present a new approach that supports model-ers in creating metamodels for diagrams they have sketched or are currently sketching. Metamodels are defined in a semi-automatic, interactive way by annotating diagram elements and automated model analysis. Our approach requires no metamodel-ing expertise and supports the co-evolution of models and meta-models.