the script is undeciphered, the writing direction has been deduced from external evidence, such as instances of the symbols being compressed on the left side as if the writer is running out of space at the end of the row. In the case of seals, which create a mirror image impression on the clay or ceramic on which the seal is affixed, the impression of the seal is read from right to left, as is this case with inscriptions in other cases. Relationship to other scripts Some researchers have sought to establish a relationship between the Indus script and Brahmi, arguing that it is a substratum or ancestor to later writing systems used in the region of the Indian subcontinent. Others have compared the Indus script to roughly contemporary pictographic scripts from Mesopotamia and the Iranian plateau, particularly Sumerian proto-cuneiform and Elamite scripts. However, researchers now generally agree that the Indus script is not closely related to any other writing systems of the second and third millennia BCE, although some convergence or diffusion with Proto-Elamite conceivably may be found. A definite relationship between the Indus script and any other script remains unproven. Comparisons with Brahmi Researchers have compared the Indus valley script to the Brahmi and Tamil-Brahmi scripts, suggesting that there may be similarities between them. These similarities were first suggested by early European scholars, such as the archaeologist John Marshall and the Assyriologist Stephen Langdon, with some, such as G.R. Hunter, proposing an indigenous origin of Brahmi with a derivation from the Indus script. Comparisons with Proto-Elamite Researchers have also compared the Indus valley script with the Proto-Elamite script used in Elam, an ancient Pre-Iranian civilisation that was contemporaneous with the Indus Valley civilisation. Their respective scripts were contemporary to each other, and both were largely pictographic. About 35 Proto-Elamite signs may possibly be comparable to Indus signs. Writing in 1932, G.R. Hunter argued, against the view of Stephen Langdon, that the number of resemblances "seem to be too close to be explained by coincidence". Theories and attempts at decipherment Decipherability The following factors are usually regarded as the biggest obstacles to successful decipherment: Inscriptions are very short. The average length of the inscriptions is around five signs, and the longest only 34 characters long, found on a copper plate belonging to the mature Harappan period. Inscriptions vary between just one and seven lines, with single lines being most