The Treaty on the Limitation of Anti-Ballistic Missile Systems was signed in Moscow on May 26, 1972. Some of the features:
USSR and US undertook not to deploy ABM systems for territorial defence
2 ABM systems were allowed: one around a national capitol the other in an area containing ICBM silo launchers
neither party could develop, test or deploy ABM systems or components which are sea,air, space or mobile land-based
national verification means allowed and not to be interfered with. No concealment.
A protocol signed July 3, 1974, reduced the number of allowed deployments from two to one — either in the ICBM silo launching area or centered on the national capitol.