From the archives...
The following question was originally posted on the (now discontinued) LOE Forum.
What do you think about the word "straight"? Does it belong in the 2% of words that does not follow the rules?
There is an IGH phonogram that says /ī/, but there is no AIGH phonogram. In fact, AIGH is only used in the word "straight" and other words where "straight" is a base word.
We teach AIGH as an advanced phonogram, since you are correct that it is used only in straight and its derivatives.
There is nothing particularly tricky about AIGH as a phonogram conceptually — it behaves just like other phonograms ending in -GH like IGH and OUGH, appearing before a T and with the "gh" being silent, and it makes a sound also represented by AI. However, we still teach it as advanced because it happens to be used only in this one base word.
A few very rare words from Scottish are sometimes spelled with AIGH — for example, "scraigh," a harsh cry or screech, and "quaigh," a kind of shallow cup. However, these terms have not entered the general English lexicon, and they are also more commonly spelled with a C: scraich, quaich.