The Federal Employees' Retirement System (FERS) is the retirement system for employees within the United States civil service. FERS became effective January 1, 1987, to replace the Civil Service Retirement System (CSRS) and to conform federal retirement plans in line with those in the private sector. FERS consists of three major components: The FERS annuity, a defined benefit plan, Mandatory participation in Social Security (most CSRS employees are not part of Social Security and do not pay taxes into the system, nor are they eligible for benefits unless they qualify under private sector employment or by being rehired and covered as CSRS with a Social Security Offset), and The Thrift Savings Plan (TSP), a defined contribution plan which operates like a 401(k). Transition from CSRS to FERS Since January 1, 1984, employees with fewer than 5 years of non-military experience on December 31, 1986, were covered under interim retirement rules under which they were covered by both CSRS and the Social Security system (commonly referred to as CSRS Offset). They made reduced payments to the CSRS (1.3 percent of earnings instead of the usual 7 percent) and contributed their full employee share to Social Security. Employees with more than 5 years of non-military service on December 31, 1986, continued under the dual benefit coverage unless they opted to switch to FERS between July 1, 1986, and December 31, 1987. Employees covered only by CSRS remained covered by it unless they opted to switch to FERS. Once an employee elected to switch to FERS the choice was irrevocable. FERS annuity Eligibility Most new federal employees hired on or after January 1, 1987, are automatically covered under FERS. Those newly hired and certain employees rehired between January 1, 1984, and December 31, 1986, were automatically converted to coverage under FERS on January 1, 1987; the portion of time under the old system is referred to as "CSRS Offset" and only that portion falls under the CSRS rules. Rehired federal employees who worked prior to December 31, 1983, and had 5 years of civilian service by December 31, 1986, can choose between remaining in CSRS or electing FERS within six months of rehire, but should the employee elect to switch from CSRS to FERS coverage, the election is irrevocable. Employees of Nonappropriated Fund Instrumentalities of the Departments of Defense and Homeland Security participate in a separate retirement system, except when retaining previous coverage