Compasses on sailboats may have additional lubber lines at forty-five degrees from the centerline.
These lubber lines may be used when sailing close hauled to see if you are on the closest course to your destination, without having to add or subtract the 45 degrees every few minutes, or recalculate your required heading every time you tack.
The main line on the compass reads your current (close-hauled) heading and the leeward lubber line will read the bearing to your destination, regardless of whether you are on port or starboard tack.
If you are sailing close-hauled with good trim and you notice that your bearing to the windward mark starts to drift outside the lubber line (angle becoming greater than 45 degrees) you are being headed, and should consider tacking.
The name comes from "lubber", a nautical term for a novice sailor or landlubber.