Psalm 127:1: unless the LORD builds the house, its builders do what?

Answer: Labor in vain

"Unless the LORD builds the house, its builders labor in vain; unless the LORD protects the city, its watchmen stand guard in vain."

“A song of ascents. Of Solomon. Unless the LORD builds the house, its builders labor in vain; unless the LORD protects the city, its watchmen stand guard in vain.”— Psalm 127:1 (BSB)