You are very close Hoodeng.

The rules are actually for where material loss is encountered.

They are very obscure, from a certain engineering trade that I stumbled into as a lad.

So it's seems the pattern makers pretend a measurement is not really what it is, so then adjust it to suit their particular needs - not an engineering rule!

Rather than measuring and then applying a correction factor, make a fake rule.

Pattern-makers - carpets, curtains, cushions? 

It is true Wideglider there is an occupation called Pattern-maker that has something to do with making clothes etc.

However an Engineering Patternmaker makes patterns usually out of timber. These patterns are used to make sand molds. The patterns are taken out of the molds and into the molds molten metal is poured to make a metal casting. That is how they make all sorts of metal items like engine blocks, pistons, cylinder/blocks, propellers for ships, pipes etc etc etc. Because different metals shrink different amounts from molten to solid you make the patterns using these contraction rules to allow for the shrinkage so that the final casting ends up the correct size.

You are very close Hoodeng.

The rules are actually for where material loss is encountered.

They are very obscure, from a certain engineering trade that I stumbled into as a lad.

So it's seems the pattern makers pretend a measurement is not really what it is, so then adjust it to suit their particular needs - not an engineering rule!

Rather than measuring and then applying a correction factor, make a fake rule.

Pattern-makers - carpets, curtains, cushions? 

It is true Wideglider there is an occupation called Pattern-maker that has something to do with making clothes etc.

However an Engineering Patternmaker makes patterns usually out of timber. These patterns are used to make sand molds. The patterns are taken out of the molds and into the molds molten metal is poured to make a metal casting. That is how they make all sorts of metal items like engine blocks, pistons, cylinder/blocks, propellers for ships, pipes etc etc etc. Because different metals shrink different amounts from molten to solid you make the patterns using these contraction rules to allow for the shrinkage so that the final casting ends up the correct size.

Nice vernier calipers Bloo.

What is that thing behind them?