I am instructed by she who has more aesthetic nouse that me that we will have horizontal shiplap cladding round most of the house but the triangles of the gables will be vertical shiplap. Any tips on how to make the transition between horizontal and vertical cladding?

I've seen a few builds round our way with some kind of break provided across the gable ends from eave to eave, even though they are vertically clad above and below this strip...no idea what material it might be or cross section though. I guess it might just be a PVC or aluminium L- or Z- shape profile? Might it have a proper name I could look up suppliers for?

thanks.

Not 100% certain however it may be some kind of cill to divert the rain water from the vertical cladding over the first horizontal cladding, which I think is how you deal with the join

Don't know if this helps, but I'm currently considering this work-in-progress detail for vertical timber above and render on carrier board below.  My understanding is that vertical timber cladding needs to be fixed to horizontal counter battens which are in turn fixed to vertical battens, whereas horizontal timber (or in my case render board) just needs to fix to vertical battens to maintain a continuous ventilation/drainage cavity. I've added a drip profile to help define the transition and throw water from the upper cladding off the cladding below.