To create Guitar Picasso made a radical leap from the sculptural tradition of modeling (carving or molding) to a new technique of assemblage. He created a first version of Guitar from cardboard in 1912, then later remade the work in sheet metal; the modern ordinariness of both of these materials is very different from traditional sculptural materials such as bronze, wood, and marble. The planes of the sheet–metal construction engage in a play of substance and void in which volume is suggested, not depicted. In a dramatic demonstration of the flexible way visual forms can be read in context, the guitar's sound hole, which normally recedes from the instrument's smooth surface, here projects outward into space.
from Focus: Picasso Sculpture, July 3–November 3, 2008
Before the twentieth century, sculpture often described the human form, and was principally an art of carving and modeling solids. In Guitar Picasso broke with these age-old traditions, examining an everyday object and initiating a new type of sculptural construction: built up from sheet metal, Guitar has no solid center but is open to space. A shallow arrangement of planes to be viewed from the front, it seems pictorial as well as sculptural, and relates to Picasso's Cubist collages of newspaper clippings and the like. This points to another departure from tradition: whereas ambitious sculptors of the period might work in bronze or marble, Picasso used sheet metal and wire—common, everyday materials, like the newspapers of the collages.
Picasso's guitar sculpture is the same size and shape as the real thing, but he shatters its form. If the front of a guitar is a plane concealing a volume, he cuts that plane away, opening up the interior as an empty box. If the sound hole is ordinarily a void, he gives it substance, turning it into a projecting cylinder (a device, Picasso said, inspired by the tubular eyes in an African Grebo mask). Viewed frontally, the cylinder's open rim becomes a line drawing of the sound hole. Here, Picasso has opened up the central core of sculpture, allowing us to see into and through it.
from The Museum of Modern Art, MoMA Highlights, New York: The Museum of Modern Art, revised 2004, originally published 1999, p. 68