[Geometric diagram]
Fig.18.

Let ABC (Fig.18.) be the triangle.

As it is given in position and magnitude, one of its sides, at least, must be given in position and magnitude, and the directions of the two other sides.

Let AB be the side given in position and magnitude.

Then AB is a horizontal line, in a given position, and of a given length.

Draw the line AB. (ProblemV.)

Let ab be the line so drawn.

Find V and V', the vanishing-points respectively of the lines AC and BC. (ProblemIII.)

[p28]
From a draw aV, and from b, draw b V', cutting each other in c.

Then abc is the triangle required.

If AC is the line originally given, ac is the line which must be first drawn, and the line V'b must be drawn from V' to c and produced to cut ab in b. Similarly, if BC is given, Vc must be drawn to c and produced, and ab from its vanishing-point to b, and produced to cut ac in a.