DOOR is a hinge word: 2 words make a compound in front of it and 3 make one after it, so it can sit in the middle of a chain and hold two compounds together at once.
Each of these makes a compound ending in DOOR.
Each of these makes a compound starting with DOOR.
DOOR is a plain object that behaves like a place. BACKDOOR and OUTDOOR position it; DOORMAN, DOORSTEP and DOORWAY treat the doorway as a spot you can stand in.
Put any word from the first list in front and any word from the second behind, and you have a three-word chain in which both neighbouring pairs join. Here are some of them.
BACK → DOOR → MAN — backdoor and doormanOUT → DOOR → STEP — outdoor and doorstepKEYSTONE is a free daily puzzle built entirely out of chains like these. You get the two ends and have to fill the two words in the middle. Three bridges a day, no account, no cookies.
Each of these has a page like this one. There is also an index of all 54.
AWAY · BACK · BALL · BED · BLACK · BOARD · BOOK · BOY · BREAK · CHECK · DAY · DOWN · FALL · FATHER · FIRE · GROUND · GUN · HAND · HEAD · HOOD · HOUSE · LAND · LESS · LIFE · LIGHT · LINE · LOAD · MAN · MOTHER · NIGHT · OFF · OUT · OVER · PAY · POINT · POST · POWER · ROOM · RUN · SHIP · SIDE · SOME · STEP · SUN · TAKE · TIME · TURN · UNDER · WALK · WATER · WAY · WOOD · WORK