IBM Peterlee Relational Test Vehicle 

A very early relational database system.  Very concise relational expressions, but not fully developed
as a delivered application.  The relational language developed for the system was called Information system base language ([ISBL]).

http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.89.5904&rep=rep1&type=pdf