The original intention of the instrument was to appeal to upright bass players. The original Jazz Bass had two stacked knob pots with volume and tone control for each pickup. Original instruments with this stacked configuration are highly valued in the vintage guitar market. In late 1961 it received three control knobs: two controlling the volume of each pickup and one the overall tone. Despite this new feature, many stacked knob models were made until about 1962. Another feature the initial models had were the "Spring Felt Mutes", which were present on basses from 1960 until 1962. The purpose of those mutes was to dampen the overtones and the sustain, and were screwed in place between the bridge and aft pickup. Those felt mutes were not a tremendous success, and were replaced by a cheaper, more simple foam mute glued underneath the bridge cover as was used by the Precision Bass from 1963 onwards. Over the following years as the use of mutes gradually declined both the Precision and Jazz Bass models eventually began to be produced without bridge/tailpiece covers.