What does the black index line on a compass serve as?

The black index line on a compass serves as a reference line for determining direction. This line is crucial for orienting the compass dial correctly with respect to the magnetic North. When taking a bearing, the user aligns the black index line with the directional arrow on the compass, which points toward the target location. This alignment allows for accurate readings of headings and enables navigation along a specific path. Understanding the function of the black index line is fundamental for effective use of a compass in various navigation scenarios.