A lightweight polyline (LWPOLYLINE) is defined in the drawing database as a single graphic entity unlike the "legacy" polyline (POLYLINE), which is defined as a group of subentities. A "legacy" polyline can be either a 2D or 3D polyline, while a lightweight polyline can only be a 2D polyline.
Lightweight polylines display faster and consume less disk space and RAM than "legacy" 2D polylines. As of AutoCAD Release 14 and later, 3D polylines are always created as "legacy" polyline entities, and 2D polylines are created as lightweight polylines, unless they are curved or fitted with the AutoCAD PEDIT command. When a drawing from an earlier release is opened in AutoCAD Release 14 or a later release, all 2D polylines are convert to lightweight polylines automatically, unless they have been curved or fitted or contain extended data (xdata).