truffers they're called here

Throughstones connect one side of the wall to the other as this picture taken from above a wall I built recently shows. The idea is that they tie together both halves of the wall and thus make the whole wall stronger. The use of 'truffers' as it is pronounced in this part of Yorkshire, near Whitby is important as most of the stone is fairly lumpy.and it is difficult to get stone to overlap from both sides.
When I lived in Ireland the use of throughstones was not always common as the stone was often irregular and/or thinner and it was a matter of course that both sides of the wall ended up with overlapping stones making the use of 'truffers' unnecessary.

A New Wall

This short section of wall was built from reclaimed building stone and is at Fylingthorpe. Typically for this area most of the stone being re-used from barns or buildings it has previously been shaped and worked. The copes (or 'corpse' as it's pronounced here are large flattish blocks spanning the finished width.