[1] The classic 1976 work of Ferdinando Gliozzi, Joël Scherk and David Olive[2] paved the way to a systematic understanding of the rules behind string spectra in cases where only closed strings are present via modular invariance.
As first proposed by Augusto Sagnotti in 1988,[3] the type I string theory can be obtained as an orientifold of type IIB string theory, with 32 half-D9-branes added in the vacuum to cancel various anomalies giving it a gauge group of SO(32) via Chan–Paton factors.
The discovery in 1984 by Michael Green and John H. Schwarz that anomalies in type I string theory cancel sparked the first superstring revolution.
The relation between the type IIB string theory and the type I string theory has a large number of surprising consequences, both in ten and in lower dimensions, that were first displayed by the String Theory Group at the University of Rome Tor Vergata in the early 1990s.
It opened the way to the construction of entire new classes of string spectra with or without supersymmetry.