The 59.94 rate is derived from the following calculations. Designers chose to make the chrominance subcarrier frequency an "n" + 0.5 multiple of the line frequency to minimize interference between the luminance signal and the chrominance signal. (Another way this is often stated is that the color subcarrier frequency is an odd multiple of half the line frequency.) They then chose to make the audio subcarrier frequency an integer multiple of the line frequency to minimize visible (intermodulation) interference between the audio signal and the chrominance signal. The original black-and-white standard, with its 15,750 Hz line frequency and 4.5 MHz audio subcarrier, does not meet these requirements, so designers had either to raise the audio subcarrier frequency or lower the line frequency. Raising the audio subcarrier frequency would prevent existing (black and white) receivers from properly tuning in the audio signal. Lowering the line frequency is comparatively innocuous, because the horizontal and vertical synchronization information in the NTSC signal allows a receiver to tolerate a substantial amount of variation in the line frequency. So the engineers chose the line frequency to be changed for the color standard. In the black-and-white standard, the ratio of audio subcarrier frequency to line frequency is   285.71. In the color standard, this becomes rounded to the integer 286, which means the color standard's line rate is  ≈ 15,734 Hz. Maintaining the same number of scan lines per field (and frame), the lower line rate must yield a lower field rate. Dividing lines per second by 262.5 lines per field gives approximately 59.94 fields per second.