The Curv-O-Mark® Aligner Base is notched at either end for ease in aligning on the centerline of the pipe and has two 25-pound pull, cast-in magnets for holder either a flat horizontal or vertical position. The base will accept either leg of a standard or combination square and will work on pipe diameters from 1" on up, requiring only the extension on the legs of the square being used to accommodate larger diameters