In a discussion in the comments section on the posts on Midtown and Downtown I made an offhanded reference to those areas being "T6". It only occured to me afterward that the t1-t6 designation might not be universally recognized. That particular designation comes from the New Urbanist "Transect".
As luck would have it John Montague Massengale posted a description of the transect on his blog Veritas et Venustas yesterday. It's a good description, with informative graphics.