This is one of the primary building functions available. It allows you to link a room to another room by adding an exit from one to the other. If a return exit name is added then that exit is added and points back to the original name. If the return name is a '.' then it will use the opposite name if possible and if not the same name. An opposite name would be east/west, or up/down, or in/out

Examples :

: *drill east 1801 .
room 1800 linked to 1801 by east and back in west direction

Note: the example above assumes you are in room 1800