The Indus Valley civilization was the first major urban culture of South Asia. It reached its peak from 2600 BC to 1900 BC roughly, a period called by some archaeologists "Mature Harappan" as distinguished from the earlier Neolithic "Early Harappan" regional cultures. Spatially, it is huge, comprising of about 1000 settlements of varying sizes, and geographically includes almost all of modern Pakistan, parts of India as far east as Delhi and as far south as Bombay, and parts of Afghanistan.
The main corpus of writing dated from the Indus
Civilization is in the form of some two thousand inscribed seals in good,
legible conditions. (In case you don't know what seals are, they are used
to make impressions on malleable material like clay.) Although these seals
and samples of Indus writing have been floating around the scholastic world
for close to 70 years, little progress has been made on deciphering this
elegant script. However, we should not blame scholars for their lack of
progress, for there are some major impediments to decipherment:
a) Very short and brief texts. The average number of symbols on the seals is 5, and the longest is only 26.
b) The language underneath is unknown.
c) Lack of bilingual texts - no at all.
But the script isn't as bad as indecipherable.
For one, even though scholars don't have long texts and bilingual texts,
they can still theorize about the language underneath the writing system.
There are several competing theories about the language that the Indus
a) The language is completely unrelated to anything else, meaning an isolate. Well, this doesn't get us anywhere.
b) The language is Indo-Aryan and therefore Indo-European. This is very doubtful, because the Indo-European Brahmi script used by Aryans, was generated about a thousand years after Indus was forgotten.
c) The language belongs to the Munda family of languages. The Munda family is spoken largely in eastern India, and related to some Southeast Asian languages. Like Aryan, the reconstructed vocabulary of early Munda does not reflect the Harappan culture. So its candidacy for being the language of the Indus civilization is dim.
d) The language is Dravidian. The Dravidian family of languages is spoken in Southern Indian, but Brahui is spoken in modern Pakistan. So far this is the most promising model. There are many Dravidian influences visible in the Vedic texts. If the Aryan language gradually replaced the Dravidian, features from Dravidian would form a substratum lexicon in Indo-Aryan. One of these features is the appearance of retroflex consonants in Indian languages, both Indo-European and Dravidian. In contrast, retroflex consonants do not appear in any other Indo-European language, not even Iranian ones which are closest to Indic.
A count of the number of signs reveal a lot about the type of system being used. Alphabetic systems rarely have more than 40 symbols. Syllabic systems like Linear B or Cypriot typically have 40 to 100 or so symbols. The third ranges from logosyllabic to logographic, running upwards of hundreds of signs (like 500 signs in Hieroglyphic Luwian). It appears that the maximum number of Indus script symbols is 400, although there are 200 basic signs (i.e. signs that are not combined from others). This means that the Indus script is probably logosyllabic, in that it has both signs used for their meanings, and signs used for their phonetic values.
Language which used the script:
Image: Indus Valley Inscription