I can’t get it to work. (this isn’t detailed enough to help: Polyline | Rhino 3-D modeling) When I invoke the “ceNter” command, the arc centre doesn’t go where I click. I get a ballooning arc with 7 nodes, all of which would need to be moved to correct it:

Hello - if you are tracing that hatch boundary, the Direction option (vertical, in Y) will set the arc going the right way.

-Pascal

If I use Direction instead of arc ceNter, I get an arc with the center where I want. If you can point me to a resource to understand how to use arc center, I’d appreciate it.

In this case, “center” is the radial center of the circular arc, not the midpoint of the arc curve, or a point on the arc.

Thanks John.
So it’s not always the centre of radius. I might have figured this out if it weren’t for this solitary explanatory sentence about “Center” on the Rhino help page on polylines. :-/:

"Center

To use the Center option, pick a center point for the arc’s radius"

From what you’ve written above, it could be any one of 4 points - the center of the arc’s radius or one of the three points you identify. Can I respectfully suggest that this explanation on the help page is misleadingly incomplete?

Even still, I don’t understand why – when I use the Center option – it constrains to the y axis (until I’ve chosen a direction, I assume?) nor how it determines the distance along that axis to place the undesired center point, given that I’ve clicked in a completely different place.

A comprehensive resource guide on how to use the polyline tool would be appreciated. I avoid it wherever I can because I can’t understand how the options work. When I have to use it, what little I have ever understood about it is by then forgotten.

I have to say that I don’t seem to be much wiser now…