Having GISJOIN fields in both the shapefiles and tables from NHGIS is so helpful for joining and using this data in GIS! I’ve wondered for a while though why the tract IDs are padded with extra 0s - e.g., the first tract in a 2010 tracts table is in Alabama’s Autaga County, state 01, county 001, and tract 020100. Throw on a G at the start, and I would expect this to concatenate into G01001020100, for G+11 digits. But you have 2 extra zeros in there - one after the state (or before the county), and another after the county (or before the tract ID), netting G0100010020100, which is G+13 digits. What’s the reasoning behind this?