Akey and scale are important components in what visual aid

Map--a map needs to key to show what symbols mean and a scale to determine distance. 

Maps commonly have a key or legend to show what different symbols used on the map mean. The scale is used to determine the map distance and transfer that to the real distance. 

The statistics for the eastern war are grim. more than three-million men died in the fighting, more than nine-million men were wounded, and every major country which participated lost its form of government.