The dual resonance model was based upon the observation that the amplitudes for the s-channel scatterings matched exactly with the amplitudes for the t-channel scatterings among mesons and also the Regge trajectory.
It began with the Euler beta function model of Gabriele Veneziano in 1968 for a 4-particle amplitude which has the property that it is explicitly s–t crossing symmetric, exhibits duality between the description in terms of Regge poles or of resonances, and provides a closed-form solution to non-linear finite-energy sum rules relating s- and t- channels.
The Veneziano formula was quickly generalized to an equally consistent N-particle amplitude[1] for which Yoichiro Nambu,[2] Holger Bech Nielsen,[3] and Leonard Susskind[4] provided a physical interpretation in terms of an infinite number of simple harmonic oscillators describing the motion of an extended one-dimensional string, hence came the name "string theory."
[5] It was even taught briefly as a graduate level course at MIT, by Sergio Fubini and Veneziano, who co-authored an early article.
[6] It fell rapidly out of favor around 1973 when quantum chromodynamics became the main focus of theoretical research[7] (mainly due to the theoretical appeal of its asymptotic freedom).