Hi,

Probably the best way to construct a perpendicular would be to:

1. Draw the horizontal line.
2. From each end of the horizontal line tack the end of a string somewhat
longer than half the length of the horizontal line.
3. Attach a pencil to the other end and construct an arc through the
horizontal line, extending above and below the line.
4. Connect the point where the two arcs meet above the line to the point
where the two arcs meet below the line and you have bisected the horizontal
line with a vertical line at 90 degrees.
5. To get as close as possible to a 90 degree perpendicular, make the
length of each of the strings as long as possible so that they will reach as
far above and as far below the half-way point in the horizontal line.

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