Early dialect of SmalltalkLanguage, and considered to be the first "usable" version of Smalltalk (Smalltalk-71 was a rather primitive prototype).  For a long time, Smalltalk-72 was a project internal to XeroxParc, and didn't became known outside until well after Smalltalk-80 (what is usually meant by just "Smalltalk") was released.  Smalltalk-72 was described by AlanKay in EarlyHistoryOfSmalltalk; and DanIngalls has recently made a re-implementation of Smalltalk-72 in SqueakSmalltalk: http://wiki.squeak.org/squeak/989

* The Smalltalk-72 Instruction Manual from 1976: http://www.bitsavers.org/pdf/xerox/alto/Smalltalk72_Manual.pdf