Under a supplementary member system (SM), which is a form of semi-proportional representation, a portion of seats in the legislature are filled by pluralities in single-member constituencies. The remainder are filled from party lists, with parties often needing to have polled a certain quota, typically a small percentage, in order to achieve representation, as is common in many proportional systems. Any supplementary seats won by a party are filled from an ordered list of nominated candidates
Unlike mixed-member proportional, where party lists are used to achieve an overall proportional result in the legislature, under SM, proportionality is confined only to the list seats. Therefore, a party that secured say 5% of the vote will have only 5% of the list seats, and not 5% of all the seats in the legislature.
The proportion of list seats compared to total seats ranges widely; for example, 18.7% in South Korea, 30% in Taiwan, 37.5% in Japan and 68.7% in Armenia.