A variation to the single tone scheme was seen in one-way paging receivers. In some two-tone sequential systems, sending 4–8 seconds of the second tone pages all receivers which have a code including the second tone. This is sometimes referred to as "long tone B". Receivers made by Plectron and often used to page volunteer firefighters use a long single tone. The decoder in the typical Plectron receiver would not decode the tone as a valid call unless it was present for at least two to four seconds, (a very long variation of the burst tone).