From IFWiki

Southwest (or south-west) is a direction. It is one of the four diagonal directions, and one of the eight compass directions. Its opposite direction is northeast.

Southwest is associated with the bottom left corner of a map; most maps are meant to be read with the north-end up. If you are facing north, then southwest is in a diagonal direction both back behind you and to your left.

Some early IF games do not support diagonal directions like southwest. In ADRIFT Generator 4.00, diagonal directions are initially disabled and must be explicitly enabled by an ADRIFT author before they can be used in a game.

The SOUTHWEST command

Typing SOUTHWEST means GO SOUTHWEST or WALK SOUTHWEST. That is, direct the player character southwestward from their current location. Assuming there's an exit in that direction and nothing is blocking or hindering the character, the PC will travel southwest to an adjacent location.

SOUTHWEST is such a common enough command that it has an abbreviation: SW.

Southwest in other languages

Some languages and regions prefer to hyphenate the names of diagonal directions. Some language library extensions support both spellings; others don't. Only the unhyphenated variants are listed here.

Dutch zuidwest zw
English southwest sw
Esperanto sudokcidenten sw, sok1
French sudouest so
German Südwesten sw
Italian sudovest so
Spanish sudoeste so
Swedish sydväst sv

1 Esperanto's words for southwest and southeast are sudokcidento and sudoriento, which means it's problematic to use so as an abbreviation for either of them. One might borrow the English abbreviation sw, but it's not ideal since there's no W in the Esperanto alphabet. There aren't enough Esperanto IF games to see any clear concensus on how to abbreviate sudokcidenten, but sw and sok seem the most likely possibilities.

Code Compare

Directions like SOUTHWEST tend to be implemented both as room properties and as objects in their own right. The SOUTHWEST command, however, tends to be parsed or reinterpreted as a GO command acting on the southwest object.

Code Compare: Southwest
ADRIFT 4:Move SouthWest to Field begin.pngClosetField end.png
Inform 6:The sw_to property and sw_obj object
Inform 7:The southwest direction
TADS 3:The southwest link property and southwestDirection object