A graphics processing system for processing polygons includes a cache with cache lines for storing data entries, each line having a tag for identifying the data stored in the line. The polygons have vertices with which pieces of vertex attribute data are associated. The system also includes processing circuitry which writes, to a line in a first set of lines, data entries associated with pieces of vertex attribute data. The pieces of vertex attribute data are associated with the vertices of a polygon. The processing circuitry also writes a tag including a polygon identifier to identify the polygon associated with the data entries to the line in the first set of lines, and writes, to a second set of lines of the cache, data entries associated with pieces of vertex attribute data. The processing circuitry also writes tags including vertex identifiers to the second set of lines to identify the vertices associated with the data entries.