=== AP Decoding With the QRA64 decoder Nico Palermo, IV3NWV, introduced a technique for decoding with the aid of information that naturally accumulates during a minimal QSO. This _a priori_ (AP) information can be used to increase the sensitivity of the decoder. When an operator decides to answer a CQ, he already knows his own callsign and that of his potential QSO partner. He therefore knows what to expect for at least 56 of the 72 message bits in a standard-format response to his call. The _WSJT-X_ decoders for QRA64 and FT8 can use these AP bits to decode messages containing them with higher sensitivity than otherwise possible. We have implemented AP decoding in slightly different ways in QRA64 and FT8. To provide some explicit examples for users, we provide here a brief description of the FT8 behavior. The FT8 decoder always tries first to decode a signal without using any AP information. If this attempt fails, and if *Enable AP* is checked on the *Decode* menu, a second attempt hypothesizes that the message contains callsigns MyCall and DxCall. If the QSO has progressed to the point where signal reports have been exchanged, a third attempt hypothesizes that the message contains the known callsigns followed by RRR, RR73, or 73. AP decoding attempts effectively set the AP bits to the hypothesized values, as if they had been received perfectly. The decoder then proceeds to determine whether the remaining message and parity bits are consistent with the hypothesized AP bits. If a codeword is found that the decoder judges to have high (but not overwhelmingly high) probability of being correct, a ? character is appended when the decoded message is displayed. Successful AP decodes are always 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, an a2 designator says that the successful decode used MyCall as hypothetically known information. [[AP_INFO_TABLE]] .AP information types [width="25%",cols="h10,