What does “twice in seven years” mean regarding checking mezuzos?

We are told to check our mezuzos twice every seven years. What exactly does this mean? If the Chachamim/Poskim had meant once every 3.5 years, would they not have said this instead?

With regard to tashmish ha-mitah, I was told "twice a week" means that there should not be a seven-day period without two episodes. Does this mean the same thing, namely that there should not be a seven-year period without two checkings, which seems to boil down to approximately every six years?

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Presumably, it means any two times in a period of seven years. – b a Aug 7 '12 at 1:36
@ba does it mean, then, that there should be no seven-year period without two checkings? Or is it a repeating seven-year cycle, where you might have seven years pass without a check? – yitznewton Aug 7 '12 at 17:26
How can seven years pass without two checkings if the halachah is that you have to check twice in seven years? They didn't say once in every 3.5 years, so it seems that you can check any two times in seven years. I don't see how seven years can pass without a check – b a Aug 7 '12 at 17:32
@ba if it's viewed as a cycle, I can check in years 2, 4, 9, 13, and 5-11 have passed with only one check. – yitznewton Aug 8 '12 at 16:55
Oh, I see your point. – b a Aug 8 '12 at 16:57

2 Answers

I have come across a minhag to check one's mezzuzos every leap year in Adar Sheini. The reason for this custom is in order to ensure they are checked twice in seven years.

The Minhag is brought in אוצר טעמי המנהגים ח"ב עמ' שצו quoting בית ברוך ח"ב קונטרס המזוזה סי' רצא סע"ג.

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This seems to be an answer to the other question judaism.stackexchange.com/questions/18311/… – Double AA Sep 9 '12 at 3:41
@DoubleAA As I see it, it's an answer to this question. That question is asking how often, the answer is twice in seven years as per Y.D. 191. This question is asking what that actually means, my answer is that some have the minhag every leap year. – Michoel Sep 9 '12 at 3:46
So what does twice every seven years actually mean? – Double AA Sep 9 '12 at 3:48
@DoubleAA :-) Year 3, 6, 8, 11, 14, 17, and 19 in the 19 year cycle. – Michoel Sep 9 '12 at 3:51
@DoubleAA True. It's a way of making sure that the minimum of twice in seven years is kept. The source I quoted bring this as the reason. – Michoel Sep 9 '12 at 3:58

Rav Moshe said every 3-1/2(ish) years.

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Source........? – Seth J Sep 19 '12 at 19:43
Do you mean R. Moshe Feinstein? – msh210 Sep 19 '12 at 21:24