Upright Basses, Mandolins and Arch-top guitars frequently have bridge adjusters. Adjusters allow the player to change the string height of the instrument. The top of an instrument can rise or lower due to temperature, humidity, aging – or even if you decide to use a very different string set.

Adjusters are made of various materials: aluminum, brass, phenol, wood, steel, etc… They can be machined in one piece or in two pieces for “English style” adjusters. The bridge sits on the round “platens”, which the threads move up and down.

Things to note about the performance of the adjusters are as follows:

• The material density, weight and shear strength DO noticeably affect the sound and performance of the instrument. For example, large brass or steel adjusters are heavy. They will reduce volume and add sustain. This would be bad for an orchestral player who mainly uses the bow; but it might be desirable for a Jazz bassist who plays mostly pizzicato, uses an amplifier and would enjoy the added sustain of the plucked note.
• Adjuster thread sizes larger than 1/4 x 20 (standard size) should only be used on bridges large enough to provide sufficient wood to support the weight of the strings. If you remove most of the wood in order to accommodate the adjuster, the bridge will crack or otherwise deform. Similarly, if the adjuster material is too weak, the adjuster may break or warp under string tension.
• The poorest adjusters come pre-fitted to a bridge blank. This makes the job of fitting the bridge to the top MUCH more difficult. To add insult to injury, many of these bridges have poorly machined, what I like to call “unintentional 2-piece” brass adjusters, which are fit into brass sleeves. You can do nothing with them.

My personal favorite adjusters are 1-piece aluminum, with hard black anodized finish. (“Any customer can have a car painted any color that he wants, so long as it’s black” – Henry Ford) The thread is 1/4 x 20, of course, strong enough to outlast the bridge and light enough to avoid influencing the sound too much. Maple or Ebony “English style” adjusters are nice, but require more work to install and require a robust bridge.