I was shown this by one of the sample machinists (Pat) at my first job in the clothing industry, where I was a "design room assistant", that is in reality - the design room dogsbody! I stayed there four years so it couldn't have been too bad - and I learnt a lot of things I still value today from those who had already spent a lifetime in the industry. And I do a good cup of tea ;)
Place your pieces right sides together as you normally do when sewing a seam. Then on the top layer turn back the seam allowance, less 1mm:
|My seam allowance is 1cm, so I fold it back 9mm|
This is how it looks when sewn:
|I hope you can make the stitching out - this was late-night-photography..... |
But look at how those blue stripes line up!
And there is the disadvantage that it isn't suitable for heavy weight fabrics because there is slightly increased bulk, but for me this is far more often outweighed by the advantages of speed, accuracy, the lack of unpicking required, and the happy memories of Pat and our workroom, when using this Magic Pat-Trick.
Anyway, I hope you find it handy one day too - happy sewing!
PS - another Pat Trick here if you are interested :)