Which sideband is commonly used for 20-metre voice operation?

By amateur radio convention, Single Sideband (SSB) voice operation uses Upper Sideband (USB) for frequencies above 10 MHz, and Lower Sideband (LSB) for frequencies below 10 MHz (with a few modern exceptions like the 60-metre band). The 20-metre band spans from 14.000 to 14.350 MHz. Since 14 MHz is well above the 10 MHz threshold, voice operations on the 20-metre band use Upper Sideband (USB). Following this standard convention ensures compatibility between stations and prevents overlapping interference. If you transmitted on LSB while everyone else was on USB, your signals would be unintelligible to each other.