Typical examples

A. Egyptian Arabic. In Egyptian Arabic, “now” is dilwa’ti; in Standard Arabic, it is ‘al’āna. Egyptian Arabic for “nose” is manaxīr; in Standard Arabic, it is ‘anf. In other cases, the Egyptian is a variation on the standard: “many” is kathirah in Standard, kətir in Egyptian. An Egyptian learns to speak, essentially, a whole new language in school.

B. Swiss German. In German-speaking Switzerland, to be a functioning person requires being bilingual in two forms of “German” that are as different as Spanish and Portuguese. High German for “drink” is trinken; Swiss German has suufe. High German has kein for “not one”; Swiss German has ke.

C. Triglossia. In particularly hierarchical societies, there can be three levels of language according to context. In Javanese, for example, there is a “middle rung” between the “highest” and “lowest” forms. Here is “Are you going to eat rice and cassava now?” on all three levels.

D. The closest equivalent to diglossia in English is the difference between such words as dine and eat, children and kids, or parcels and bags. Imagine if differences like these applied to most of the words in the language!