This post is a little bit of an unusual one.  There is an option in the preferences called “Allow 3D sketching of lines and splines”.  The option is located under the Design tab.  You can see the location below.

So, if you haven’t noticed this option, you’re not alone.  Most have not.  Before we explore the how and why, I think we need to establish a sketch fundamental (to confirm everyone is on the same page).  ALL sketches in Fusion 360 are also 3D in nature.  Below is an active sketch with line and spline elements.

With the same sketch active, simply selecting the Move/Copy tool, you can drag sketch elements into 3D space.  Please note, that at the moment, this does have an impact on dimensions.

This can be very, very powerful.  You can move items and points into 3D space.  Below you can see a spline point moved into 3D space.

Ok, now that we have the baseline covered, lets talk about what the preference does.  Here you can see that I have an active sketch on top plane.  I have built a line on this active sketch.  In addition, I previously built a quick surface using a plane tool in form/sculpt.  The result is a surface body.

Now you can see that I started a spline on the active sketch.  I started it at the end of the line segment.  Clearly, the active selection end of the spline is on the active plane.

Time for the magic.  As I mouse over the corner of the surface, the cursor changes to a square.  This tells me that I have left the active plane and now will snap to the corner of the surface.

On this last step, I simply make the spline tangent to both the surface edge and the sketch line.

This 3D sketch that is anchored to the surface and the sketch can now be used in a myriad of ways.  How about a solid or surface pipe?  A loft profile or guide curve?  You got it and others as well.  I hope you enjoyed this little tidbit.  Keep in mind, this is a very powerful tool.  You can easily accidentally select off the current sketch plane with this preference on.  Keep that in mind.  Lastly, you can also turn this feature on and off in the sketch palette.

Last option, 3D Sketch.

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