errors in excavation or recording. Urban archaeological sites are complex affairs, often generating thousands of units of archaeological stratigraphy (contexts). It is of even more vital importance when excavating such sites to compile the matrix as the excavation progresses. Such sites by definition produce multi-linear sequences of succession and to date the best way to get a handle of these sequences is to compile the matrix by hand, based on the drawings and the context sheets. This ensures an internally consistent record and that the complexity of the site is given due regard. Computer programmes do exist which can aid the production of a matrix, though at the moment these tend towards articulating linear sequences rather than multi-linear sequences. The Harris matrix is a tool that aids the accurate and consistent excavation of a site and articulates complex sequences in a clear and understandable way. Harris matrices play an invaluable role in the articulation of sequence and provide the building blocks from which higher order units of stratigraphically related events can be constructed. Example Take this hypothetical section as an example of matrix formation. Here there are twelve contexts, numbered thus: A horizontal layer Masonry wall remnant Backfill of the wall construction cut (sometimes called construction trench) A horizontal layer, probably the same as 1 Construction cut for wall 2 A clay floor abutting wall 2 Fill of shallow cut 8 Shallow pit cut A horizontal layer A horizontal layer, probably the same as 9 Natural sterile ground formed before human occupation of the site Trample in the base of cut 5 formed by workmen's boots constructing the structure wall 2 and floor 6 is associated with. The order in which these events occurred and the reverse order they should have been excavated with would be demonstrated by the following Harris matrix. Completed matrix The later a context's formation is, the higher it is in the matrix, and conversely the earlier it is, the lower. Relationships between contexts are recorded in the sequence of formation, so even though wall 2 is physically higher than other contexts in section, its position in the matrix is immediately under backfill 3 and below floor 6. This is because the formation of the backfill and floor happened later. Also note the matrix splits into two parts below the construction cut 5. This is because the relationships across the section have been destroyed by the