Sketch and Layout Curves

If you select one or more end points of sketch curves, the curves are concatenated into a single spline. Neighboring sketch curves are merged into one selectable item with an underlying spline. The spline is not displayed until the concatenated curve is modified:


Example:
Let's look at the following sketch curves:
One of the rectangles isn't closed. If you fill this rectangle, the loop is automatically closed and it becomes a surface:

The gap in this loop is small enough for the Fill tool to automatically close. Larger gaps will not be closed. If a gap is less than 1.5 times the length of the minor grid spacing on the sketch grid, the edges are extended to close the gap. If the gap is larger, a message appears in the status bar and the gap's endpoints flash.

The Fill tool also created surfaces from the other closed loops of sketch curves. The darker shaded areas in the image above show where the surface parts overlap. If you move the rectangle, you can see the surface that was created automatically:

The result only has edges for the sketch curves that were not selected, because the selected curves were used to create a separate surface.

Now let's go back and close the open rectangle, and let the Fill tool automatically create surfaces without selecting any edges:

This surface has edges for every closed sketch curve. The same thing automatically happens if you go from sketch mode to 3D mode.

If you select all the sketch curves, then click Fill, you get a surface without any interior edges:
You can also fill layout and sketch loops:
And a loop of non-tangent 3D curves: