The J Register is one of the specialised registers within the relay computer. It can hold two 8-bit values (J1 and J2) and can read (but not write) from the data bus.
Additionally it can also write (but not read) a 16-bit value to the address bus combining the J1 and J2 8-bit registers together. Typically it is used to store the address for a jump on the GOTO instruction.
Below is a list of the blog articles relating to this card (in reverse date order):