Also known as ‘Edge’ constraint in SAP2000, the auto line constraint automatically connects adjacent elements that do not share common joints, such as a coarsely meshed shear wall intersecting with a finely meshed floor slab. Instead of having to manually re-mesh areas/shells or frame elements to create shared mesh points on the adjacent elements, the auto line constraint ‘zips’ them together using displacement constraints. By default, the auto line constraint is turned on in ETABS, but off in SAP2000 (user can modify these defaults). For more information please go to: http://www.csiberkeley.com/Tech_Info/Line%20Constraint.pdf.