[Geometric diagram]
Fig.15.

Let the horizontal line AB (Fig.15.) be given in position and magnitude. It is required to find its dividing-points.

Find the vanishing-point V of the line AB.

With center V and distance VT, describe circle cutting the sight-line in M and N.

Then M and N are the dividing-points required.

In general, only one dividing-point is needed for use with any vanishing-point, namely, the one nearest S (in this case the point M). But its opposite N, or both, may be needed under certain circumstances.