Little Sammy Sneeze (1904-06) by Winsor McCay Although historically the form dealt with humorous subject matter, the scope of the art form has expanded to encompass all genre, leaving artists free to explore their own self expression. Comics are seen as a low art, having established as a form in the late 19th and early 20th century. While preceding works are seen as sharing certain techniques, most notably the conveying of a narrative through imagery, most commentators are agreed that the form was established in the the humorous cartoons and comic strips found in newspapers and magazines of the late 19th and early 20th century, and was fuelled by the boom in publishing technology. Early precursors include Trajan's Column and the Bayeaux Tapestry, as well as works by William Hogarth, Rodolphe T?pffer, Wilhelm Busch and George Cruikshank. Discussion of the form is even more recent, especially in the United States; although early criticism can be traced back to the 1920s, it is only in the latter half of the 20th century that serious examination of the form was begun. Definitions of the form are rather loose in their approach; some claim its printed format is crucial, some emphasize the interdependence of image and text, and others the sequential nature of the images. Different conventions were adopted around the globe, from the Manhua of China to the manga of Japan, the comic books of the United States and the anthology comic magazines featuring a variety of strips in Europe, initially established by the Ally Sloper comic paper. The comic strip established itself on the pages of newspapers through the success of Outcalt's Hogan's Alley and Yellow Kid, and was soon adopted elsewhere. Although practitioners can eschew any formal constraints, they often use particular forms and conventions to convey narration and speech, or to evoke emotional or sensual responses. Devices such as speech balloons and boxes are used to indicate dialogue and impart establishing information, while panels, layout, gutters and zip ribbons can help indicate the flow of the story. Comics use of text, ambiguity, symbolism, design, iconography, literary technique, mixed media and stylistic elements of art help build a subtext of meanings. Similarly, the partnering of words and pictures can create a synergy of expression through the manner of their combination which can either enhance or subvert the meaning of each partner's individual contribution. The twentieth century saw an explosion in the genres that comic strips and comic books tackled, expanding from their humorous beginnings to take in action, science fiction, whimsy, mystery, romance, superhero, autobiography and reportage. The comics form was also utilised for educational purposes and by the late twentieth century stories of an extended length were being published as graphic novels. The most common forms of printed comics are comic strips (most commonly four panels long) in newspapers and magazines, and longer comic stories in comic books, graphic novels and comic albums. In the first two forms the comics are secondary material usually confined to the entertainment sections, while the latter consist either entirely or primarily of comics.