Decoders look for a valid first tone followed by a valid second tone within a defined length of time, (a "time window"). For example, a decoder detecting a valid first tone might allow up to 2 seconds for a valid second tone to be decoded. If no valid second tone is decoded within 2 seconds, the decoder resets and waits for another valid first tone. A widely varied set of tone plans or schemes are used for these systems. Some tone plans use tone frequencies which are close or overlap with tones used by other coding plans. For example, one plan might use very narrow filters and specify a tone of 702.3 Hz. Another may use a simple filter of capacitors and inductors and specify a tone of 700 Hz. A decoder might not be able to tell the difference between these two tones because they are so close in frequency. Systems generally use tones off of a single, designed tone plan. Individual tone plans are engineered to avoid overlapping or nearby tone frequencies that may cause falsing. Some systems use CTCSS subaudible tones as the tones composing the two-tone sequence. For example, a two tone sequence might consist of 123.0 Hz followed by 203.5 Hz. On FM two-way radios, tones are usually sent at a level that equals two-thirds of system deviation. For example, in a ±5 kHz deviation system, the tone encoder is set to produce 3.3 kHz of transmitter deviation, (modulation,) or less. Because the tones are audible, in systems with solid received signals, tone levels are sometimes set lower so that radio users are not forced to listen to them at a high level. Keeping the tone modulation below system maximum preserves the clean sine wave produced by the encoder. Sending digits at higher levels causes the transmitter's circuits that are designed to prevent over-modulation to distort or clip the waveform of the tones. Distorted wave forms may not decode properly or may include harmonics that cause falsing. Tones are usually sent at a minimum of 500 milliseconds (ms) to 3 seconds (3,000 ms) in length. Radios with two-tone sequential decoders may monitor all system traffic or remain muted until called, depending on the system design. When the radio receives the correct tones in the proper sequence, it may momentarily buzz or sound a Sonalert. An indicator light may turn on and remain latched on. In most systems, the radio's receive audio would latch on if normally muted.