A bassbar is a supporting brace on the inside of the top of a violin. If you look in the bass “ff” hole you should be able to just see part of it. The bassbar runs lengthwise to the top and should run just inside the bass bridge foot, mirroring the position of the soundpost. The exact position of the bar to the bridge has a huge impact on the sound of the instrument, but unlike the soundpost, which is held in place only by tension, the bassbar is glued into place. A bassbar must be chalk-fit perfectly to the inside of the top to work properly.
When I am preparing to install a bassbar I find the bridge placement and check with a small brass gauge to make sure the bar is in exactly the correct place. After I am satisfied with the placement I fit small temporary blocks called cleats to the inside of the top next to the bar. These cleats will ensure that as I fit the bar it will maintain the same position. I rub soft chalk on the top under the bar and then gently put the bar in place and move it slightly back and forth. Where the bar contacts the top, chalk will highlight the area. When the chalk is marking the entire surface of the bar, it fits perfectly.
I use many clamps and light pressure to glue the bar in to avoid damaging the unvarnished surface of the top.
Bassbars have changed a great deal since the days of the conception of the violin. In the old days the bass bars were shorter and thinner, but modern string tension and the demands of virtuoso players have required a modern bassbar which is up to the task. Some old instruments had bassbars which were not separate pieces of wood, but most all of these have been removed now.