When I am running this command, I am usually using it as from “PointOnCurve”.
And then sometimes it works correct, but most of the time - not.
Even thou I am sure that curve is planar.
It works normally if not picking “PointOnCurve” option.
Does anyone else experiencing this? Maybe I am doing something wrong?
When you say “not working correctly”, what are the symptoms? --Mitch
As usually I forgot to mention it. Sorry.
It just “fly” freely around, even thou planar mode is active. Not only in XY projection but in XYZ.
Ortho should constrain the line the way you are looking for, I think.
-Pascal
it kinda does 
Thank you, Pascal
yep, it does that instead of staying planar to cplane as sometimes expected.
for setting the second point, reference other geometry to determine which direction the perp line will be drawn. (near snap may come in handy for it).
can you show a typical situation where you’re having problems drawing the perpendicular line?
jeff_hammond,
of course I could use reference geometries. I could build a tangent line first and than rotate it 90 degres.
But why use so much commands if there is one instead.
i meant reference something already in the model…
for instance, if i want to draw a perpendicular line on the plane of this arc, i can set my cursor on the arc and the perp line will go in the proper direction:
if i want the line perpendicular from the curve on a plane parallel to the cplane, i push the shift key (to engage ortho)…
if i want it to go in some other direction, i’d need to do some temp geometry to define the plane… because a perpendicular line can be drawn in any direction within the whole 360º of the point… you have to somehow tell it which direction to go… either using ortho, existing geometry, or temporary geometry… (or maybe some other method depending on what exactly you’re trying to accomplish… if you show a situation you’re having problems getting it to work right, i’m sure if you upload the example here- someone will be able to be more specific to the situation)
I think I didn’t understood the concept of this option from the beginning.
But now as jeff_hammond explained it - now I get it.
Thank you
it might also be important to point out that all of the red lines in the following image are perpendicular to the point on the arc… (plus the infinite amount of lines that could fit in between these ones ;).)… so yeah, in some form or another, you have to tell rhino which one you want it to draw.
Thanks jeff_hammond 