Draw a line $l$ and a point X on it. Through X, draw a line segment $\overline{XY}$ perpendicular to $l$. Now draw a perpendicular to $\overline{XY}$ at Y. (use ruler and compasses)

We may draw a line segment $\overline{XY}$ perpendicular to $l$ though X as follows:

(i) Draw a line $l$ and take point X on it.

(ii) With X as the centre and a convenient radius, draw an arc intersecting the line $l$ at two points A and B.

(iii) With A and B as centres and a radius greater than XA, draw two arcs, which cut each other at C.

(iv) Join XC and produce it to Y. Then XY is perpendicular to $l$.

(v) With Y as the centre and a convenient radius, draw an arc intersecting XY at two points D and E.

(vi) With D and E as centres and radii greater than YD, draw two arcs that cut each other at F.

(vii) Join YF, then YF is perpendicular to XY at Y.