These were often used in the blues for allowing the guitar player to slide from note to note on the higher sounding strings while playing individual notes on the lower sounding strings:

   Bottlenecks

Explanation:  Bottleneck or slide guitar as it is also known, is the technique of playing the guitar as described above, and some hard object is used as a bottleneck or slide. That hard object can be a steel bar that pulls on the finger, which crosses the strings on the neck of the guitar and slides along the neck. Also, that hard object could also be the neck of the bottle from which the name of this musical accessory came from, because blues guitarists who initially played just around the bars used what they had in that environment, bottles.