The Free On-line Dictionary of Computing (30 December 2018):

IBM 709

    A computer made by IBM oriented toward
   scientific work.  The 709 had the same basic architecture as
   the IBM 704 but with many I/O and performance refinements
   over the 704.

   The IBM 709 (like the 704) had 36-bit memory words, 15-bit
   addresses and instructions with one address.  A few index
   register instructions had the infamous 15-bit decrement field
   in addition to the 15-bit address.

   The IBM 7090 was a transistorised version of the 709.

   [Difference between 704 and 709?]

   (1999-01-19)