Two-tone sequential, also known as 1+1, is a selective calling method originally used in one-way, tone-and-voice paging receivers. Many companies have their own names for two-tone sequential options. General Electric Mobile Radio called it Type 99. Motorola called it Quik-Call II. For example, the encoder sends a single tone followed by 50 to 1,000 milliseconds of silence and then a second tone. 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.