How does slide guitar work in Mississippi Delta blues?

In Mississippi Delta blues, the slide guitar technique is a defining element used to create a plaintive, whining sound that evokes the human voice.12

To perform this technique, musicians press a hard object—typically a glass bottleneck or a metal object like a knife—against the guitar strings. By sliding this object along the fingerboard, players are able to bend notes smoothly, creating expressive melodic phrases.123

This style is often facilitated by the use of "open tunings," where all strings are tuned to the pitches of a single chord, allowing the player to produce full chords and melodies across the neck with the slide. In the context of Delta blues, these guitar phrases frequently serve as a "response" in a call-and-response pattern, interacting directly with the singer's vocal "call". The technique is believed to trace its origins to the diddley-bow, an instrument of African derivation.123