My disc is sitting right on the OB line, half in and half out. Is that OB?

The short answer: the disc is in-bounds

If any part of the disc is still over in-bounds ground, the disc is in-bounds. Half-and-half — part of the disc over the grass, part over the line — is in-bounds, with no penalty. Under 806.02, a disc is OB only when it is clearly and completely surrounded by the out-of-bounds area.

The painted line is out-of-bounds

The OB line itself — paint, string, or any other marking — is part of the OB area. A disc that is only touching that line, with no part over in-bounds ground, is not “half in.” That disc is completely surrounded by OB, so it is out-of-bounds.

Any part of the disc that is only on the line is over OB. The disc is in-bounds only when some part of it is still over in-bounds.

When it really is OB

The disc has to be clearly and completely inside the OB area, with the boundary running all the way around it. Then it is out-of-bounds, you add one penalty throw, and you take relief as described in our OB and relief FAQ.

Two things to remember on the line

First, the marked line is OB, and OB rises as a vertical plane from that line, so what the disc is resting on does not decide the call — whether any part of it is still over in-bounds does. Second, do not touch a borderline disc until the group has looked at it. If the thrower moves the disc before the group rules, it is treated as OB. The PDGA’s OB questions and answers cover the common edge cases.