Inter-record gap (IRG)
The blank stretch of tape between two records that gives the drive room to speed up and slow down between reads.
On a start-stop tape drive the mechanics cannot halt on a single byte. So between each written record the drive leaves an unmagnetised gap — about 0.6 inch on classic 9-track tape — where the tape can brake after one record and reach speed before the next. The gap is pure overhead: many small records can put more tape into gaps than into data.
The remedy was blocking, packing several logical records into one physical block with only a single gap around it, governed by a blocking factor. A distinct gap, the tape mark, separated whole files. Streamers shrank the problem but never to zero.