To find places on the map, we use card slots or card boxes. There is a letter above and below the card and a number on the right and left of the card. There is a letter and a number in the registers behind a map. These refer to (verwijzen naar) the card boxes. This way you know which part of a map you have to look at.
If you have to tell on which map you have found something, you always have to mention the card slot behind the number.