For over 30 years, French voters have sat down in front of the radio or television after the last polling stations close at 8:00 pm (1800 GMT) to hear very accurate result predictions.
These are based on actual votes cast rather than the potentially troublesome exit polls used in many democracies.
The campaign officially ends on April 21, with candidates barred from making public statements and no opinion polls allowed to be published, in order to prevent others' opinions or preliminary results swinging voters.
But polling organisations are authorised to be present when votes are counted after polling booths in the countryside and smaller towns close at 6 pm. They then publish predictions that are embargoed for French media.
The results, available before 7pm, are historically accurate to within less than one percentage point and are released by French media, principally television channels when the French gather to watch the evening news at 8pm.
While it would be illegal to publish the embargoed result predictions in France before 8pm, foreign media, notably francophone outlets in neighbouring Belgium and Switzerland, have no such compunction.
Add the speed of the Internet to the equation, along with France's 23 million Facebook and three million Twitter accounts, and the law banning result predictions before 8:00 pm appears increasingly unworkable.
French media contacted by AFP, including key broadcasters, newspapers and magazines, said that they would abide by the embargo rule.
"One can criticise the law but we will respect it as long as we're a media based in France," an unnamed journalist at the website of conservative newspaper Le Figaro told AFP.
Nevertheless, many media are prepared to revise their position if other French media break the embargo.
Left-leaning daily Liberation said it "reserves the right" to publish results on its website "if the gap (between lead candidates) is big and if the sources are reliable," said editor Nicolas Demorand.