The maximum usable frequency for any time through a 24-hour period can be determined by examining the green line. The frequency required to reach the receiver must exceed the frequency indicated by the red line or the signal will be reflected by the E-layer and will be more heavily absorbed. The optimum working frequency (or FOT) is indicated by the yellow line and is defined as the frequency corresponding to 85% of the MUF. Frequencies between the magenta colored line and the yellow line are often the best frequencies (most reliable) to use for the given transmitter and receiver.