Family Income Supplement was a means-tested benefit for working people with children introduced in Britain in 1970 by the Conservative government of Edward Heath, effective from August 1971.
Half of the amount by which the claimant's income fell below £15/week was paid, plus £2 for each additional child, to a maximum of £3/week (revised to £4/week).
In addition, those in receipt were given entitlement to free school meals and passported to the NHS Low Income Scheme.
There was a maximum payment to prevent abuse from claimants or employers deliberately reducing or understating earnings.
Each extra £1 a week in gross earnings would reduce weekly Family Income Supplement by 50p, on top of which most claimants would be subject to income tax and National Insurance Contributions of at least 30p, which produced a very high marginal withdrawal rate.