Isometric sketching: a clevis
This video inspired me to suggest a session for the USITT National Conference. We called it "Analog Sketching for the TD." It shows the drawing of a cleve, from start to finish, using markers. One of the keys to the artist's process is turning the paper while s/he draws. One particular technique I've picked up is using a finger to help guide the pen during shading. The finger works as a stopper of sorts.
youtube user vcTECsketch