What does the compass card do in a magnetic compass?

The compass card in a magnetic compass serves the primary function of displaying the heading relative to magnetic north using the lubber line. The lubber line is an optical reference line marked on the compass that indicates the aircraft's heading. When the pilot aligns the lubber line with the compass card, it effectively shows the direction in which the aircraft is pointed concerning magnetic north.

This functionality is essential for navigation, as pilots rely on the compass to maintain course and make directional adjustments during flight. Accurate readings from the compass card allow pilots to interpret their heading and make informed decisions while flying, especially when in low visibility conditions or when other navigational aids may not be available.