Okay, I have yet another possibly-obvious question.
There are multitudes of tutorials on how to use mattress stitch to seam, and they’re great. But they all stop with “continue in this way” until you’re done- but, when you get to the end of the seam, what do you do? Do you weave in the end of the yarn you’re seaming with? Do you tie it off to something?
So, I guess my question is - how do you secure the yarn you use to seam something once you’re finished with the actual seam?
Thanks!
I don’t know if it’s considered the ‘right’ way or not, but I just bring the yarn up through and back down through the edge a couple of times.
anouck…sooner or later you have to cut the yarn 
Like Ingrid I tend to weave the yarn back down through what I have done already on the seam a few inches. I probably tend to be over fussy on this at times but I’ve never had anything unravel yet
I guess I do have a tendency to look back at all those hours of knitting and think that it’s holding itself together so precariously - and then I start obsessing about seams coming undone, sweaters unravelling, snagging yarn… (and admit to myself that I’m a little ocd sometimes)
But thanks, this advice helps - if it’s good enough for Ingrid, it’s good enough for me. 
I’ll knot it, take it back up through the stitches and then bring it back down about halfway and snip off the end.