This list reflects the order in which United States senators began their service. This is different from seniority lists, which are maintained by the party caucuses (for the purposes of committee assignments) and by the Senate Committee on Rules and Administration (for the purpose of office space assignment). Members who began service on the same day are listed alphabetically. Up-to-date biographical information for all senators is available at https://bioguide.congress.gov. A key to party abbreviations is at the end of the list.