tape is wide, accommodating 6 data tracks, 2 mark tracks, and 2 clock tracks, with data recorded at roughly 350 bits per inch (138 bits per cm). Each track is paired with a non-adjacent track for redundancy by wiring the tape heads in parallel; as a result the electronics only deal with 5 tracks: a clock track, a mark track and 3 data tracks. Manchester encoding (PE) was used. The clock and mark tracks are written only once, when the tape was formatted; after that, they are read-only. This meant a "drop-out" on one channel could be tolerated; even a hole punched through the tape with a hole punch will not cause the read to fail. Another reason for DECtape's unusually high reliability is the use of laminated tape: the magnetic oxide is sandwiched between two layers of mylar, rather than being on the surface as was common in other magnetic tape types. This allows the tape to survive many thousands of passes over the tape heads without wearing away the oxide layer, which would otherwise have occurred in heavy swap file use on timesharing systems. The fundamental durability and reliability of DECtape was underscored when the design of the tape reel mounting hubs was changed in the early 1970s. The original machined metal hub with a retaining spring was replaced by a lower cost single-piece plastic hub with 6 flexible arms in a "starfish" or "flower" shape. When a defective batch of these new design hubs was shipped on new DECtape drives, these hubs would loosen over time. As a result, DECtape reels would fall off the drives, usually when being spun at full speed, as in an end-to-end seek. The reel of tape would fall onto the floor and roll in a straight line or circle, often unspooling and tangling the tape as it went. In spite of this horrifying spectacle, desperate users would carefully untangle that tape and wind it laboriously back onto the tape reel, then re-install it onto the hub, with a paper shim to hold the reel more tightly. The data on the mangled DECtape could often be recovered completely and copied to another tape, provided that the original tape had only been creased multiple times, and not stretched or broken. DEC quickly issued an Engineering Change Order (ECO) to replace the defective hubs, to resolve the problem. Eventually, a heavily used or abused