Triangulation is a technique for how to draw that is the single most important drawing technique you will ever learn. If you master this drawing technique you will be able to draw anything you want.
The contour drawing technique however, as taught in many popular books on how to draw, can actually hinder your ability to draw well. This is because it trains you to narrow your attention to a specific point on a curve and follow that curve and its changes of direction. This is training you to do the exact opposite of what you should be doing. If you wan to learn how to draw accurately, you must always focus on seeing the big picture and the large relationships even while you are drawing a specific part. You start with these big relationships and first get them accurate. Then you progressively move to smaller and smaller relationships.
I have taught students how to draw using this method who had absolutely no drawing practice before and who were starting from ground zero. In two hours they learned the basic procedure for how to use this method. After one hour a day practice for one week they all learned to draw quite well, even though at the beginning of the week they could only draw simple stick figures and always drew with the proportions way off!
With triangulation, you can even draw the difficult drawing subjects such as heads and figures. However, if you want to draw figures and portraits really well (including animals) then you must also study structural anatomy. This is a much longer course of study and requires a lot more knowledge. Having said that though, if you master this triangulation technique, your figure drawings will be at least 95% accurate.
In triangulation, you compare the angles of the sides of a triangle formed by three points on your drawing with the same triangle on the subject. This results in accurate proportions. It is the single most important drawing technique to learn.
It is rather tedious to start with, but becomes very fast once you are proficient at it. Eventually you will be able to do it by eye without holding up a straight edge to compare angles.
You will practice the key skills in triangulation in Lesson: B12 Drawing Triangles