The Piggyback Twistor (PBT) is an electronically writeable nondestructive-readout memory that has been built and tested in sizes up to 300,000 bits. The storage and sense functions are performed by thin magnetic tapes spirally wrapped in coincidence on a copper wire. The mode of operation is by linear selection using a ferrite core array for word access. “Ones” and “zeros” are of opposite polarity at the output terminals to ease the discrimination problem. This paper discusses details of the construction and operation of a piggyback memory. The magnetic properties of the two tapes are considered, and the theoretical read and write field limits are then examined. The the results are given for a 4,096-word 54-bit memory.