Separator Line

Alternatively for the bevel line look you can use a label with its Autosize turned off, border style set to Fixed3D, and height set to 2. This allows you to use docking and anchor points. It also works for vertical lines. The only annoying point is selecting and moving.

For solid lines of various widths use the same trick with a blank label and appropriate background color, height/width set to desired thickness.

Limited of course to horizontal & vertical lines. Diagonals you need to use GDI+.

Well, not that I know of directly; you can't put

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in a win forms label control and get a horizontal line (it would be nice if you could).  You can draw the lines using GDI, or you can use an image of the line.  If you cannot group and box up your controls using group boxes here is a cheap trick that might work for you.  Add two labels to the form, set the Bold property to true on both and fill the labels with underscore characters to get the width you want.  Make one white and one black.  Click on one and use the arrow keys to drive it down to point where the lines very nearly overlap.  The result looks like a 3D line which you could use as a separator.