a unit of <#sheetstamp=sheet stamps> as produced on a printing press that is subdivided into <#pane=panes>. A sheet contains two or more panes. Early sheets contained two panes. Most 20th century sheets contained four panes. Modern sheets may contain six or more panes. Although there are exceptions, sheet stamps are generally sold to the public only in pane format, not in sheet format. In popular usage the term "sheet" is incorrectly used for a "pane."