Writing provides a means for personal reflection, thinking, creativity, meaning-making and sharing, as well as complementing other modes of communication in a world of multimodal texts. While writing in the digital age has become increasingly fast-paced and exposed to global scrutiny, being able to write efficiently with correct spelling, grammar and punctuation remains a critical part of being a literate writer. This article uses data from 819 Australian primary school students to explore the relationship between three language conventions, namely spelling, grammar and punctuation as measured by the National Assessment Program Literacy and Numeracy (NAPLAN) Language Conventions Test, and the quality of written composition, as measured by the NAPLAN Writing Test. Results indicate that spelling, grammar and punctuation jointly predict written composition achievement with spelling as the main predictor. Implications for the educational practice of writing in the contemporary context are discussed, emphasising the importance of spelling in relation to writing and how instruction in spelling, during senior primary school, appears to be critical for written composition improvement.