While most instruments – acoustic or electric – fall into the category of monophonic (capable of playing only one note at a time) or polyphonic (capable of playing multiple notes), most Arp synthesizers (the Odyssey, for example) could play two notes at a time, and thus are considered duophonic. However, the limitation on these synths was that there was only one ADSR envelope controlling both notes. Thus, if you played middle C on the keyboard and then a few seconds later added a major third, that note did not re-trigger the envelope. However, if the attack was set to a slow value, and both notes were triggered at the same time, it would sound more natural, since the envelope was applied to both notes at the same time. While Arp used the duophonic capability of its synths as a marketing point in its literature, players quickly realized that having only one envelope for both notes was a pretty big limitation to overcome.