How do guitar slides create the plaintive sound heard in Mississippi Delta blues?

In Mississippi Delta blues, the plaintive, whining sound is achieved by using a hard object—traditionally a glass bottleneck or a metal implement like a knife—to press against and slide along the guitar strings.12

By moving this object along the fingerboard, musicians are able to bend notes continuously, which creates a smooth, expressive quality that is often described as being evocative of the human voice. This technique is frequently paired with "open tunings," where all strings are tuned to the pitches of a single chord, allowing the player to produce fluid melodic phrases that serve as a response to the singer's vocal "call". This interplay between the voice and the guitar is a hallmark of the emotional intensity that defines the Delta blues style.324