The origin of the term "footage" is that early 35 mmsilent film has traditionally been measured in feet and frames; the fact that film was measured by length in cutting rooms, and that there are 16 frames (4-perf film format) in a foot of 35 mm film which roughly represented 1 second of silent film, made footage a natural unit of measure for film. The term then became used figuratively to describe moving image material of any kind. Television footage, especially news footage, is often traded between television networks, but good footage usually commands a high price. The actual sum depends on duration, age, size of intended audience, duration of licensing and other factors. Amateur film footage of current events can also often fetch a high price on the market – scenes shot inside the World Trade Center during the September 11, 2001 attacks were reportedly sold for US$45,000. Sometimes film projects will also sell or trade footage, usually second unit material not used in the final cut. For example, the end of the non-director's cut version of Blade Runner used landscape views that were originally shot for The Shining before the script was modified after shooting had finished.