The letter 'yud' has traversed a confused, almost schizophrenic path. It originally indicated the consonantal sound /y/, like our English letter "wye." Three thousand years ago, the ancient Hebrews invented another use for it, adding a vocalic role to the letter. So for most of its history it has also represented the sounds /i/ as in the "ee" of "see," and /ei/ as in the "ay" of the American pronunciation of "day." (In fact, it was in part this doubling up of purpose, using 'yud' and two other consonants as vowels, that led to widespread writing and perhaps to the ubiquity of the Bible.) When the 'yud' was first introduced, spelling was largely a matter of personal preference. Some words, like 'eifo' (where) seem to have required a 'yud': 'alef-yud-peh-heh'. But frequently, a scribe would add a yud to a word only for extra clarity. Thus, "David" (that's "King David" to you and me) was usually just 'daled-vav-daled', but sometimes a 'yud' got thrown in: 'daled-vav-yud-daled'. The vocalic 'yud' proliferated widely over the course of the first millennium BCE. By the time the Dead Sea Scrolls were composed after 200 BCE, the letter 'yud' appeared in words like 'mei'ot' (hundreds), even though we never find that spelling in the Bible. With so many vocalic 'yuds' floating around, scribes may have been hard pressed to make it clear when they had a consonant in mind, so they started writing two 'yuds' for the single consonantal sound, just to be clear. With modern Israel and the Israeli Hebrew that it brought, things got even more complicated, for three reasons. First, the Academy of the Hebrew Language formalized two distinct spelling systems, one to be accompanied by the dots and dashes we call the vowels, and another for vowel-less writing; today we're talking about the latter. In that system, Israelis needed a way to distinguish between /i/ and /ei/; that's the second complicating factor. And thirdly, a new sound needed attention. The academy decided that the sound /i/ should almost always be written with a 'yud'. (One exception it makes is the word 'mila' that means "word," 'mem-lamed-heh'. It never has a 'yud', lest it get confused with the word 'mila' that means "circumcision," 'mem-yud-lamed-heh' - though I can't imagine a context where that mistake might be made.) Writing a 'yud' for /i/ caught on, but it became even more widespread than it was supposed to be. For example, the academy didn't want people to use a 'yud' in words like 'michtav', letter, but most people put one after the 'mem' anyway. Israelis therefore came to associate the single 'yud' with the sound /i/, leading to the second problem: they needed another way to write /ei/, particularly in foreign words. Their solution was a double 'yud'. So, for example, the American state of Maine is spelled 'mem-yud-yud-nun'. Similarly, people often spell David Broza's first name 'daled-yud-yud-vav-yud-daled'. (He himself does not.) Without the extra two 'yuds', the word looks like dah-VEED, not DAY-vid. The third problem was the new sound /ay/ as in the American word "eye." We'll use /ei/ for the American "(d)ay" and /ay/ for "eye," noting as we do that the difficulty of keeping these two sounds straight in English writing matches the difficulty that Hebrew has. The obvious way to write this /ay/ sound in Hebrew was a double 'yud'. So the English word "mine" would be spelled 'mem-yud-yud-nun' in Hebrew, even though that also spells "Maine." Tal from Haifa reminds me that the social networking site "myspace" in Hebrew is 'mem-yud-yud-samech-peh-yud-yud-samech'. The four letters 'mem-yud-yud-lamed' spell "mail" (which means "e-mail" in Hebrew) and also "mile." By this point modern spellers had the same problem that first surfaced some 2,000 years ago: It was hard to indicate the simple consonant /y/. A single 'yud' didn't do it, that letter most naturally being read as the vowel /i/. So frequently (the official rules are complicated and modern practice is in flux) a double 'yud' is used for this purpose, too. For example, 'binyan', building, is spelled with a double 'yud'. So there are four potential uses of the letter 'yud': the consonant /y/, the vowel /i/, the vowel /ei/ and the vowel /ay/, but except for the second of those, each usage gravitates toward a double letter. The idea was to avoid confusion by using a double 'yud' instead of a single one. Instead, spelling is once again a matter of personal preference. The writer teaches at HUC-JIR in New York City.