Steps of construction:
(i) Draw a line segment ¯¯¯¯¯¯¯¯AB of any length and mark a point M on it.
(ii) With M as centre and a convenient radius, draw an arc intersecting the line AB at two points C and B.
(iii) With C and D as centres and a radius greater than MC, draw two arcs, which cut each other at P.
(iv) Join PM. Then PM is perpendicular to AB through the point M.