The number two is derived from an Old English word (twa), which comes from Proto-Germanic (the “mother” of all Germanic languages). The ultimate source is the Proto-Indo-European reconstructed word *duwo (“two”), which originated the same word in many other languages, such as duo in Latin (English words like duo, duet, duplicate, duplex, for instance, come from this Latin root).
Can you imagine that the Armenian word yergoo (երկու “two”) also comes from the same Proto-Indo-European root *duwo? It means, then, that there is a family relationship with two!
The relationship is complicated, but real, even though both words do not seem to have anything in common.
In linguistics, particularly, appearances tend to be misleading. As we have seen many other times, the relation of two words may be obscured by odd linguistic patterns and the passing of time. The Proto-Indo-European initial *p yielded Armenian h; the typical example is *pater > hayr (հայր “father”). Thus, in a similar fashion, *dw became erk (երկ) in Classical Armenian (now pronounced yerg / երգ in Western Armenian). Although none of the various explanations for the evolution *dw > erk has been universally adopted, the relation between both roots is generally accepted.
Since two and yergu are far cousins, so are the words double (which has a French origin, but ultimately comes from the same *duwo) and grgeen (կրկին “double”). The latter is actually krkin in Classical Armenian (from erk + kin).
The relationship between two and yergu brings another interesting couple to the fore: twin and yergvoriag (երկուորեակ “twin”). Our interest derives from the fact that it is not uncommon to replace them in colloquial language with another, actually false couple: twin and zooyker (զոյգեր).
True, zooyk (զոյգ) is a word relatively close to twin; it means “couple,” “pair,” “duo,” and also “double.” However, be advised that every time someone refers to a couple of twins as zooyker (զոյգեր), he or she is on the wrong track. Unlike twin, the word zooyk does not have the meaning of two people born together from the same mother, and the use of zooyker (plural of zooyk) looks like a literal copy of English twins.
Incidentally, if you meet “two couples of twins” (i.e. four people) in a place, it would be ridiculous to call them… yergoo zooyk zooyker / երկու զոյգ զոյգեր. The only Armenian word for twin is yergvoriag, and thus, the accurate phrase would be yergoo zooyk yergvoriagner (երկու զոյգ երկուորեակներ).
In order to make it crystal-clear, a) if you get married, you form a zooyk; b) if you have twins, they are yergvoriagner.