JT9 shares many characteristics with the modes JT65 and JT4 made popular in WSJT. All three are designed for making reliable, confirmed QSOs under extreme weak-signal conditions. They use nearly identical message structure and source encoding. JT65 was designed for EME ("moonbounce") on the VHF/UHF bands and has also proved very effective for worldwide QRP communication at HF; JT4 is used mainly on the microwave bands. In contrast, JT9 is optimized for HF and lower frequencies. JT9 is about 2 dB more sensitive than JT65A while using less than 10% of the bandwidth. World-wide QSOs are possible with power levels of a few watts and compromise antennas. Several dozen JT9 signals fit easily into a 1 kHz slice of spectrum.