From the archives...
The following question was originally posted on the (now discontinued) LOE Forum.
ne ces si ty - Why does not SI say /sh/ or /zh/ in accord with the spelling rule hint for the phonogram SI, "at the beginning of any syllable after the first one"? Perhaps Spelling Rule 5, "I and Y may say /ĭ/ or /ī/ at the end of a syllable," can help here. But when I see S and I together I don't know when I should see the multi-phonogram SI or I should unglue SI to see two single letter phonograms, S and I.
Good question. First, it's important to keep in mind that the fact that a multi-letter phonogram exists does not mean that every time those letters are found together they are working together as that phonogram. It only means they can work together to say a unique sound that is different from the sounds they say individually. Sometimes they are just saying their individual sounds one after another, not saying the different sound they make as a group. For example, consider the words ferret (which uses E-R, not the phonogram ER), reality (which uses E-A, not the phonogram EA), and fear (which uses EA-R, not the phonogram EAR).
So, as you discovered, the word necessity does not contain the multi-letter phonogram SI; in this word s says /s/ and i says /ĭ/, some of their usual individual sounds, just as they do in sit or silly or adhesive.
Now, as to how you know this when you are reading a word.
Sometimes, the written word doesn't tell you for sure. You have to try both options and check them with the English words you know and how they are pronounced to figure out which of the options is being used, and if it's not a word in your spoken vocabulary you may need to look it up. For example, someone who had never heard reality pronounced would not know for sure when reading it whether it was /rē ă li tē/ or /rēl i tē/, because they wouldn't know whether the E and A are functioning separately or together. For someone who knows the word reality this is not a problem, but someone who doesn't would then need to look it up in a dictionary to find out for sure.
On the other hand, necessity actually gives you another clue, because every syllable in English has a written vowel. SI is a multi-letter consonant. In mission, where SI says /sh/, the vowel is O (saying a schwa sound). In necessity, there is no other vowel in the second syllable, so we know that the i is likely to be functioning on its own as a vowel. Rule 18 teaches us that SI can be used only at the beginning of a syllable; a vowel has to come after it in the middle.
This means that if there isn't another vowel after it, you know it's not going to be the phonogram SI; it has to be single S and single I. However, if there is another vowel after it, you still have to try both options to see which one is a real word; for example, in enthusiasm, S and I are working separately, not functioning as one phonogram to say /sh/.