The base path (URI) to which the reference is relative is the location where the referencing file would hypothetically be written, were it to be unencapsulated and written to a file, such that the set of related encapsulated documents share the same base path.

The path shall not begin with a slash or two slashes.

The path shall not refer to a higher level in the file system hierarchy (i.e., use of ".." is not permitted).

File name extensions corresponding to executable file types (exe, dll, etc.) are not permitted.

Embedded white space is not permitted.

"matlist.mtl"

"materials/matlist.mtl"

"./materials/matlist.mtl"

"file:///matlist.mtl" (not a relative reference)

"/matlist.mtl" (initial slash not permitted)

"//matlist.mtl" (initial slashes not permitted)

"c:/matlist.mtl" (not a relative reference)

"../matlist.mtl" (".." not permitted)

"mat list.mtl" (embedded spaces not permitted)

"materials\matlist.mtl" (backslash not permitted)