Purpose: This study examined dimensions of written composition by using multiple evaluative approaches such as an adapted 6 + 1 trait scoring, syntactic complexity measures, and productivity measures. It further examined unique relations of oral language and literacy skills to the identified dimensions of written composition. Method: A large sample of 1st-grade students (N = 527) was assessed on their language, reading, spelling, letter writing automaticity, and writing in the spring. Data were analyzed using a latent variable approach, including confirmatory factor analysis and structural equation modeling. Results: The seven traits in the 6 + 1 trait system were best described as two constructs: substantive quality and spelling and writing conventions. When the other evaluation procedures such as productivity and syntactic complexity indicators were included, four dimensions emerged: substantive quality, productivity, syntactic complexity, and spelling and writing conventions. Language and literacy predictors were differentially related to each dimension in written composition. Conclusion: These four dimensions may be a useful guideline for evaluating developing beginning writers' compositions. [This article was published in "Journal of Speech, Language, and Hearing Research" (EJ1029471).]