It was originally 60 fields per second (30 interlaced frames per second), on black and white TVs. The highest frequency generated in a black and white TV was t

It was originally 60 fields per second (30 interlaced frames per second), on black and white TVs. The highest frequency generated in a black and white TV was the horizontal scan rate, which was a multiple of the frame rate. With the addition of the NTSC color signal, which used a 3.579545 MHz carrier wave, the highest frequency generated in the TV became much higher. To keep the hardware simple, all lower frequencies were still divisors of the highest frequency, now color carrier wave. For the frame rate, it came out to 59.94 fields per second.