Tongue and Groove
Tongue-and-groove boarding joins each board to the next with a projecting tongue on one edge and a matching groove on the other. The joint allows the timber to move seasonally without opening a visible gap, which is why it was the standard method for lining walls, ceilings and cupboard backs long before sheet materials existed.
The visible detail is usually a small bead or a V-groove machined along the joint. That detail is not decoration for its own sake: it puts a deliberate shadow line where an accidental gap would otherwise appear, so the wall reads as a designed rhythm rather than as boards that have shrunk apart.
Board width dates a scheme more than any other variable. Narrow boards read as utility and as the service parts of older buildings; wide boards read as later and more domestic. Mixing widths within a single room almost always looks like a repair.
Painted, the boarding becomes a texture that catches raking light along its length, which makes it useful on walls that would otherwise be blank, and difficult on walls where a strong side light will exaggerate every imperfection in the run.