Which mode is most commonly used for voice communications on the 17- and 12-meter bands?
Why Amateur SSB convention puts lower sideband on 160, 80/75 and 40 meters, and upper sideband on 20 meters and higher, which includes the 17- and 12-meter WARC bands. This is custom rather than FCC rule, but everyone follows it so contacts are intelligible. USB is also the standard on 60 meters and on VHF/UHF weak-signal SSB.
Watch out Lower sideband is the convention only on the bands below 10 MHz (160, 80/75 and 40 meters); pick it on 17 or 12 meters and you would sound like gibberish to everyone listening.
20 meters and up, use upper. Below 20, go lower.
17 and 12 meters live upstairs with 20 meters, above the 10 MHz landing, so Upper sideband. Only 160, 75 and 40 go Lower. Pool tell: 'Suppressed sideband' and 'Double sideband' are never the answer anywhere in this pool.