图中显示了一个四边形 PQRS。

1. Construct two points equidistant from $S$ and $R$ using compass arcs.

Set the compass radius to more than half of $SR$. With centre $S$, draw arcs above and below $SR$. Without changing the compass radius, with centre $R$, draw arcs to intersect the first pair of arcs.

2. Draw the perpendicular bisector and mark its intersection with $PQ$.

Use a ruler to join the two arc-intersection points. This line is the perpendicular bisector of $SR$. Extend it if necessary until it meets $PQ$. Mark the point of intersection on $PQ$ and label it $Z$.