Having finished the back and front of the gansey, I had expected the shoulders to ultimately be grafted together but without the aid of diagrams (to be fair, there is a picture of the finished shoulder at the end of the pattern but it didn't entirely help) I was scratching my head a little over the written instructions before realising that this was a saddle shoulder, which I have come across before.
Googling 'shoulders for Beatrice Gansey' didn't turn up anything helpful, so I'm posting these pictures in case they help someone at the same stage.
This is my first finished shoulder, which I'm pretty proud of!
So after finishing the front and back, those have markers placed to mark the shoulders and neck (in my case, pattern A, 27/43/27). The pattern asks you to cast on 17 on a separate needle. This is where they'll fit, 'bridging' a space between the front and back.
First, join that last cast-on stitch by knitting into the first stitch of the front/back (they're the same) and slip the last cast-on stitch over it. (makes a nice secure join).
turn the work, slip that stitch you've just made and then purl across those new co stitches. Purl the last of those together with the first stitch from the other shoulder.
Then turn the work again and start working chart D.
A tip that would have been helpful is that the first stitch of each knit row of chart D is the one that you slip, and the last on that chart is the one that you 'slip, knit, pass slipped stitch over' together with the next shoulder stitch. On the purl rows, again the first stitch of the chart is the one that you slip, and the last stitch of the chart is the one that you p2tog with the next shoulder stitch.
This way, you 'eat up' the shoulder stitches of the back and front one by one until you reach your markers.
I now know that Googling 'saddle shoulder' would have turned up some videos (some of these use a provisional cast-on which does complicate things a little further, but it may be a good idea if, like me, you don't like picking up).
