1st and 2nd fix carpentry are construction terms which refer to the different stages of structural works.
First fix involves the initial works in a build including wall studs, roof struts, doors frames, floor joists etc.
The second fix carpentry is completed at the final stages of a build. These include skirting, architrave, doors, window boards etc. Joinery involves wood working techniques to create beautifully designed components such as staircases, cabinets, furniture, doors and shelves.