3 ms·See linked Wikipedia article for explanation.by Drdrdrq 9y agoSee linked Wikipedia article for explanation.nilved 9y agoNormally a post like that is accompanied by a link to a Wikipedia article.jacquesm 9y agohttps://en.wikipedia.org/wiki/Interlock_(engineering) https://en.wikipedia.org/wiki/Interlock_(engineering)Drdrdrq 9y agoTrue, sorry. User wpietry already posted the link, I just wanted to point out that the whole story is available there, which includes answer to his question.