I combined four shapefiles into one with union. All attribute columns and properties are the same amongst the four shapefiles. When the union finished, the attribute tables joined horizontally instead of vertically. (When you scroll down the attribute table there are long lines of (-1) negative ones and zeroes with the next attribute table appended below but tabbed over.) I did this union process to 3 other shapefiles previously with no problems in my attribute table. Is there another tool recommended to combine shapefiles? Or does someone have input on what could cause this glitch?