Which mode is most commonly used for voice communications on frequencies of 14 MHz or higher?

By convention, amateur radio operators use Upper Sideband (USB) for single sideband (SSB) voice communications on frequencies of 14 MHz (the 20-meter band) and higher. This rule of thumb generally applies to all amateur bands above 9 MHz. This standard originated from the design of early SSB transceivers. The internal mixing schemes used in these legacy radios naturally produced Lower Sideband (LSB) on the lower frequency bands and USB on the higher frequency bands. While modern digital and software-defined radios can easily transmit either sideband on any frequency, the amateur community maintains this historical gentleman's agreement to ensure compatibility. If you try to use LSB on 20 meters, listeners using the standard USB setting will hear unintelligible, inverted audio.