the belt of a rectangular two - reel tape cartridge is stretchably entrained around the tape packs , a single belt - driving roller , and four idler rollers . each idler roller is positioned near a corner of the cartridge so that the belt has an angle of wrap of at least 150 degrees at the periphery of each of the tape packs . such angle of wrap permits the cartridge to employ magnetic recording tape that has a smoother backside and yet avoids read / write errors at high tape speeds .