How do reasons combine? How is it that several reasons taken together can have a combined weight which exceeds the weight of any one alone? I propose an answer in mereological terms: reasons combine by composing a further, complex reason of which they are parts. Their combined weight is the weight of their combination. I develop a mereological framework, and use this to investigate some structural views about reasons. Two of these views I call “Atomism” and “Wholism”. Atomism is the view that atomic reasons are fundamental: all reasons reduce to atomic reasons. Wholism is the view that whole reasons are fundamental. I argue for Wholism, and against Atomism. I also consider whether reasons might be “context-sensitive”.