Word-for-word, être dans les clous means to be in the nail, as in the metal pins used to hang pictures.
What does this have to do with plans to trial a new kind of health pass in south west France?
Hotel union, Union des métiers et des industries de l'Hôtellerie (l'UMIH), have made 20,000 health pass bracelets available in venues near Bordeaux to try to make going to restaurants easier for customers.
The concept means customers can pick a favourite restaurant or bar and show proof of vaccination and their identity once to the manager, in order to be given a permanent bracelet to wear allowing re-entry without having to show a health pass again.
The experiment is legally complicated, and the prefecture has been working with the union to make sure the experiment is dans les clous.
The expression être dans les clous means to follow the rules or the timescale.
Up until 1950 in France, zebra crossings were not marked by white lines but by parallel lines of nails hammered into the ground, leading from one pavement to another.
Dans les clous means to walk between the nails, or to follow the rules of the road by crossing in the correct area.
Originally applied to pedestrians, the expression is now used to refer to projects, plans, ideas and all sorts of things that are within the rules.