Standard 17.5 mm magnetic film uses 35 mm magnetic film which has been slit lengthwise into two equal widths and lengths before use. The "heads" of one-half of the 35 mm donor become the "heads" of one 17.5 mm length while the "tails" of one-half of the 35 mm donor become the "heads" of the other 17.5 mm length. 17.5 mm magnetic film was used as a secondary "shop standard" at Paramount and Universal for location dialogue recording ; it was most often run at 45 feet/minute, one-half of the usual 35 mm magnetic film speed, thereby achieving a 4-to-1 increase in economy although at a significant sacrifice in sound fidelity, but adequate for monophonic dialogue. For stereophonic dialogue, conventional 35 mm magnetic film was used.