A string retainer bar can be fitted to most guitars.
It is fitted behind the nut or lock nut.
The purpose of the string retainer bar is to fhold the strings down causing better contact with the nut or locking nut.
This will in turn improve the tone of the strings.
The strings pass first over the nut, then under the retainer bar and finally on to the machine heads.
The strings retainer bar is held down by two screws.