The liners are 1/2" x 1 1/2" pieces of poplar that are glued to the inside perimeter of the case. They act as a ledge for the soundboard to sit on. In this photo, the spine liner is being glued to the spine side. The bentside and tail liners will also have the hitchpins buried in them and serve to hold the hitchpins tightly. Since the bentside and tail liners will be under string tension, they must fit against the case sides perfectly and be well glued to them. The liner must be bent to fit the curve of the bentside perfectly. The wood is soaked in water overnight and then placed in the steambox for 1/2 hour. When removed from the steambox, it is clamped into position in the case and allowed to dry. Once dry, it will retain the shape of the bentside - almost. There is some spring back that will affect the fit against the case side. A second method is to cut kerfs almost all the way through the liner which will allow it to bend freely. This is the method I use and it always results in a perfectly fit liner against the bentside. It is also possible to combine the two methods - use the steambox method to get the fit close, and add kerfs where needed to perfect the fit.