Why was the word for king 'rei' changed to 'rey'?

Question

The Spanish word 'rey' used to be spelt rei (as it still is in some old place names, e.g. Plaça del Rei).

Why did it change from one spelling to another?

Answer 1

The rei spelling occurs in many Western Romance languages, but not in Castilian. The most ancient epic poem that remains to us in Old Spanish, El Cantar de mio Çid, notably has these lines:

En yra del rey Alffonsso yo sere metido.
Si con uusco escapo sano o biuo
Aun çerca o tarde el rey querer me ha por amigo;
Si non quanto dexo no lo preçio un figo.
Ffablo Myo Çid, el que en buen ora çinxo espada:
Martin Antolinez, sodes ardida lança:
Si yo biuo doblar uos he la soldada.

So rey was already spelled rey back in the Twelfth Century. The same is true of plaza, although other words did use the cedilla:

Vio que entrellos e el castiello mucho auie grand plaza:
Mando tornar la senna, apriessa espoloneaua:
Ffirid los caualleros, todos sines dubdança.

In contrast, it is now spelled rei in most other Iberian Romance, including in Asturian, Aragonese, Catalan, Galician, and Portuguese. Old Portuguese shared the rey spelling with Castilian, but that is no longer used today.

Shifting Orthographies

Let’s remember that just as the Latin alphabet did not originally have the letter v (let alone w) but only u, so too did it have no letter y or j, only i. This meant that the u and i had to serve as both vowel and consonant.

Another important point is that there was no “fixed orthography” for many centuries. Different scribes spelled things differently, and no one thought anything of this. Notice how in El Cid above, you see the spellings biuo for modern vivo and myo for modern mío.

But even after the printing press, there was a great deal of variation. It was not so long ago that San Isidro was San Ysidro, and even the Castilian conjunction y was of old written e (as in Portuguese) and much more recently sometimes seen written as i (as in Catalan).

The website www.curiosidario.es “Curiosidario: Curiosidades de la Lengua Española” has an article on the Historia de las letras in which they write:

En el español antiguo la representación del sonido [i] la compartían la i (conocida como i corta o media), la llamada baja o larga (origen de nuestra j, como vimos en su apartado) y la I (conocida como alta). A su vez, la i corta o media y la i baja o larga (j) podían hacer oficio de consonantes, invadiendo el terreno de la y, con lo que la y se vio obligada a invadir el terreno vocálico de las otras. Debido a tal confusión, no era poco frecuente ver, por ejemplo, la palabra viejo escrita también vieio o vjeio; o mayor y maIor; o sin y sjn; o isla eysla; o Pompeyo, Pompejo y Pompeio. La solución empezó a darse con la aparición de la imprenta, pero tardó siglos en llegarse al acuerdo actual: i es vocal en todos los casos, i griega para la conjunción copulativa más para el sonido [i] al final de palabras que acaben en -ái, -éi, -ói y frecuentemente en -úi (Uruguay, guiriguay, ley, doy, muy).

Así pues, desde 1726 la y se convirtió oficialmente en la conjunción copulativa del español, menos cuando la siguiente palabra empieza con i, en cuyo caso se sustituye por una e.

So for example, from Gonzalo de Berceo we read in Milagros de Nuestra Sennora from the thirteenth century, we read “nunqua varón en duenna metió maior querencia” for what we would now write “nunca varón en dueña metió mayor querencia”.

There are many, many more examples of writing being different in past centuries than it is today. There are many reasons for this, however, with various stories to be be had for each.

Answer 2

Notice that Plaça del Rei is Catalan, not Spanish (cf. Plaza del Rey).

As for why Catalan rei retained a more conservative orthography (given both forms derive from the Latin regis), I think this is due to Catalan (among other Romance languages) historically maintaining the influcence of Romance for longer than Castillian.

Castillian, due to its half a millennia politico-economic-military expansion along the peninsula, suffered more changes than Catalan.

It's a poor resume, but 800 years of history is hard to summarize. I suggest taking a look at Wikipedia's History of Catalan for further detail.