This is a tongue & groove detail using Windsor One T&G product. Any seems would be covered in the paint prep with bondo. Different manufacturers offer different lengths but sometimes you can get away with full runs soffit to soffit to avoid seems. With T&G there is a high paint maintenance as the wood settles, lacquer paint will often pull or crack and needs routine touch up. Something to keep in mind. You can always use a flat stock (MDF) and route the grooves in to avoid separation. Good luck!

I must have this beautiful bedroom! Great job!

excellent information about the tounge and groove, so if one used semigloss would you still need to touch up often? thanks

Beautiful ceiling, I am getting ready to do this on an older kitchen ceiling. Priming all edges of the pine boards. My question is, would you strip the ceiling under the bead board with a 1x or use a s4s board to insure a nice level surface?