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In English slang, talking double-dutch or Double Dutch means talking apparent nonsense or in a confusing and contradictory way. The term may be used to describe a speech or policy statement so inconsistent or difficult to follow that a hearer/reader might say "it's all double-dutch to me". Holland used to be a rival to England and the Dutch phrases were a form of contempt.
The phrase 'going double Dutch' means using two types of contraceptive at the same time (a practice common in the Netherlands): the man wears a condom, the woman swallows an oral contraceptive. This is supposed to be the safest type of sex, and is recommended by a lot of official institutions, because it simultaneously protects against pregnancy and venereal diseases (like AIDS).
A mathematical analysis of the use of two contraceptives at once shows this to be a good practice. In theory, the reliability of the best contraceptives approaches or exceeds 99%. In practice, the reliability is usually much less. It is estimated that for most people who carefully use them, quality contraception has a failure rate of about 3%.
Given a failure rate of 3%, or 3 in 100, using dual contraception provides virtually certain contraceptive protection. If two forms of contraception each have failure rates of 3%, then the failure rate of the pair used in unison is the product of the failure rates of the methods used separately, or 9 in 10,000. Thus the risk of unwanted pregnancy drops from 300 in 10,000 to 9 in 10,000.
For the inexperienced or those who might not be as careful in their use, use of two simultaneous forms of contraception may mean the difference between successful prevention of unwanted pregnancy and the need to decide what life changes to make as a result of an unplanned one.
Double Dutch also means playing jump rope with two ropes and two people jumping simultaneously; the involves four people total: two jumping and two turning the ropes.
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