As it pivots up and down, the main bar changes slope. The
resulting lines orient to a remote vanishing point that would be
otherwise difficult to establish. E.G. Lutz’s book Practical
Drawing includes the diagram above. Concept artist and
illustrator Craig Elliott discovered an antique centrolinead
dating from around 1890 (below). Note how the Y-legs branch from
a point along the top line of the ruler. He reports: “I did a
little building facade and some windows to test out the
Centrolineaid. It is such a breeze, even better than using a
long ruler! I set the map pins at 3 inches from the HL and the
blades at 30 degrees. I did the verticals with a t-square and
the 30-60 triangle shown. I think two or three of these would
make darn quick work of a two- or three-point perspective
drawing on a quite small drawing board. This board is about 24 x
32. Another benefit of this tool seems to be that it doesn't pop
off the pin all the time like a regular ruler would- it is very
steady on the 2 pins. For the pins or nai