The fixings choose the fixed point when nobody else does

January, a covered shed at Tuzla, a cabin opened up for other work. A fingernail run along the line where a stone counter meets its timber upstand drops in, and it catches at the after end, at the forward end and everywhere between, at one width.

That evenness is most of the diagnosis. Poor workmanship is worst at one end, and an impact crosses a face; a line that opened because two materials moved against one another opens down its whole drawn length, by season.

Something already took the decision

Every junction aboard has a free length and a fixed point: free length is how much of a run is left able to move, and the fixed point is where it is held and cannot. Neither gets added later. Both are in the timber and the adhesive from the moment the piece is bedded, whether or not anybody chose where to put them.

If nobody settles which of two materials is held and which is free, the decision is still taken — by the glue, by the bed, by the screws, and by which side happens to be stiffer. It is taken the crude way, because a rigid bond on both sides is what the quickest fixing produces. Fastened at two points, the free length between them is nothing: the movement goes into the material and something gives — the bond, the substrate, or the lacquer. Fastened at one, the rest is free and the movement arrives at the far end, where somebody has to have decided what becomes of it.

So the question at such a line is never whether it has a fixed point, but whether a person chose it or the fixings chose it by default. Where this journal has written about interior failures the cause has been a defect in a part; here every part was right and the junction opened anyway.

The yard already works this way where the joint is obvious

None of the physics is new, and most of it is in print here. Deck caulk carries a bond breaker at its base because a seam bonded on all three faces has no free length in which to stretch. A structural glue line's thickness follows from the fact that glass and an aluminium superstructure do not expand alike. A fastener makes a fixed point wherever it lands, which is why a skirting screwed through the floating layer undoes the pads beneath it.

Nobody mistakes a caulk seam for decoration where the line is read as a joint. Inside, the same two quantities are more often left to the fixings.

Where it turns up inside

• Stone landing on a timber carcase. The stone does not care what the humidity does; the carcase cares a great deal. Held hard on both sides, the stiffer wins. A compliant bed and a slip plane put the fixed point on the stone and leave the carcase free. The mitre then has to be worked out of the solid.
• Metal landing on veneer, at a stair nosing or a handrail capping. Temperature moves one and humidity the other, out of phase. A screw at close centres holds the run everywhere, which is holding it nowhere in particular; one fastener locating it, with slotted holes down the rest, puts the fixed point where a person put it. Flushness goes.
• Lacquer returning into a leather-lined reveal. Two substrates, one tensioned over a former, and the smallest step telegraphs down the line. A shadow gap keeps the two from meeting at a line at all, so neither has to carry the other. The clean butt somebody drew does not survive it.
• A solid rail returning round a corner, where a shaped length meets a straight one. The grain runs along one and across the other, so the same change in moisture moves them by very different amounts. Left slightly apart with the ends square, the two lengths give the movement somewhere to go; butted tight, the joint takes it.

Each of them is a decision about which side is held. Drawn without that decision, they are just details.

What the parts cannot settle between them

The stone, the carcase, the capping and the hide each pass against their own drawing, and an opened line between them is not a fault in any one of them: it is a quantity that lives in the gap and appears on nobody's drawing.

Some of those parts are made here and some arrive on a lorry; stone is quarried and cut by a supplier, as it is anywhere. What eleven in-house workshops on one payroll change is the fortnight after that line opens: the joiner, the finisher and the upholsterer answer to one employer, so the people who made the parts that meet there can stand at the open line on the same morning rather than by letter.

Evidence that somebody chose

A section drawn through the junction says which side is held: the ordinary evidence that a person made the choice rather than leaving it to the bed and the screws.

An elevation shows where the line falls across a face. A section shows what the two materials are doing either side of it, which is where yacht interior movement joints are settled, long before anybody runs a fingernail along one.

An allowance has units

An even hairline inside the twelve-month warranty is what the argument turns on: a defect, or a movement the design permitted. An allowance somebody chose has units on a drawing; one the fixings arrived at has none, so nothing later can say whether the hairline is inside it.

Reading it in the season she is in

A door that binds in August is read differently from one that binds at one corner all year, and a junction reads the same way. That a dimension carries a date and a condition, on blocks or afloat is settled in this journal already, along with the reason: a reading whose condition nobody wrote down compares with nothing later. What a junction adds is the season. Write the month down with the reading, whether she is in a shed or afloat and whether the heating has been running.

This is also where the dry fit stops. A dry fit closes every joint and checks every reveal along its length, assembled in the shed on a jig taken off the vessel and checked under lighting matching what will be installed, then taken apart again: a week's work, in one week's weather. That settles geometry. A season is not geometry, and one week of weather cannot stand in for a year of it.

A line read once has not been tested at all, whatever width it read at. The fixings and the timber are working on it either way, and whatever nobody decided for them, they will have decided by the time she is read again in the other season.

Written at the yard in Tuzla. Movement figures for a given stone, adhesive, hide or alloy belong to their makers and are not quoted here; whether any junction crosses a rated division belongs to her flag and the attending surveyor; and nothing described above is a tolerance this yard promises.