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.