// Status: edited === AP Decoding The _WSJT-X_ decoders for FST4, FT4, FT8, JT65, and QRA64 include procedures that use naturally accumulating information during a minimal QSO. This _a priori_ (AP) information increases sensitivity of the decoder by up to 4 dB, at the cost of a slightly higher rate of false decodes. AP is optional in FT8, JT65, and QRA64, but is always enabled for FT4 and FST4 when decode depth is Normal or Deep. For example: when you decide to answer a CQ, you already know your own callsign and that of your potential QSO partner. The software therefore "`knows`" what might be expected for at least 57 message bits (28 for each of two callsigns, one or more for message type) in the next received message. The decoder's task is thus reduced to determining the remaining 15 bits of the message and ensuring that the resulting solution is reliable. AP decoding starts by setting AP bits to the hypothesized values, as if they had been received correctly. We then determine whether the remaining message and parity bits are consistent with the hypothesized AP bits, with a specified level of confidence. Successful AP decodes are labeled with an end-of-line indicator of the form `aP`, where `P` is one of the single-digit AP decoding types listed in Table 1. For example, `a2` indicates that the successful decode used MyCall as hypothetically known information. [[FT8_AP_INFO_TABLE]] .FST4, FT4, and FT8 AP information types [width="35%",cols="h10,