Objects may be either "direct" (embedded in another object) or "indirect". Indirect objects are numbered with an "object number" and a "generation number" and defined between the codice_12 and codice_13 keywords if residing in the document root. Beginning with PDF version 1.5, indirect objects (except other streams) may also be located in special streams known as "object streams" (marked codice_14). This technique enables non-stream objects to have standard stream filters applied to them, reduces the size of files that have large numbers of small indirect objects and is especially useful for "Tagged PDF". Object streams do not support specifying an object's "generation number" (other than 0).