What part of a map explains the markings and symbols?

The legend is a vital part of any map as it provides explanations for the various symbols and markings that appear on the map. It acts as a key, allowing the user to understand what different colors, lines, shapes, and symbols represent, such as roads, landmarks, bodies of water, and other features. Without the legend, it would be challenging to interpret the map correctly, as the symbols may not be intuitive to every user. This makes the legend essential for effective navigation and understanding of the geographical information portrayed in the map.

The other parts mentioned, like the scale, primarily help in understanding distances; the index is a reference tool for locating specific items on the map; and the grid serves to organize the map into sections for easier location finding, but none of these fulfill the function of explaining symbols and markings, which is the primary role of the legend.