The array is a powerful tool that builds students' understanding in multiplication. Students' interactions with the array changes through the course of an instructional sequence, which can be viewed as a process of "reification." In this paper, I report the findings of a research study conducted with 45 Year 5 students in Sydney. The study explored students' changing use of the array through the course of an instructional sequence on multi-digit multiplication. Design Research methods were used to track students' use of different forms of the array and the functions that these forms served. Three key stages were identified in the process of reifying the array in multi-digit multiplication.