From about 1984 onwards, an original theatrical one sheet poster is typically printed as a double-sided poster. On the other hand, reproduction one sheet posters are usually printed on one side only, and may run either at the original one sheet size of 27x40 inches, or at a smaller dimension of 24x36 inches.
Distributors print original movie posters for promotional purposes at cinemas or theatres. In the strictest sense, original movie posters are not meant for sale to the public and should either be returned to the distributor or destroyed after a theatrical run, but somehow they have always found a way to collectors’ hands. That is why they are considered rare movie memorabilia or collectibles.
Reproduction posters, however, are usually licensed by distributors and printed in large quantities to be sold directly to the public. Often, they feature alternative designs to their original counterparts, and in some cases, their artwork can be much more appealing. Regardless, originals are likely to fetch a higher value than reproductions due to their scarcity and quality of print.