When I was relatively new to Spanish, one of my teachers explained to our class that voseo was a special feature of Argentinian Spanish. Voseo is the use of vos, with its associated verb forms, instead of standard Spanish tú, as an informal pronoun meaning “you”. So “you speak” is vos hablás instead of tú hablas, “you are” is vos sos instead of tú eres, and so on.
Years later I learned that voseo isn’t limited to Argentina, nor to its neighboring countries in South America. It’s found in several countries in South America, and also in parts of Central America, including El Salvador. (I seem to be running into a lot of vos-using salvadoreños lately, both at home and out of town.) Below is a map showing where vos is used in Latin America.
By the way, this is my second-favorite voseo map. My favorite is on p. 156 of Christopher Pountain’s Exploring the Spanish Language; do a “Search inside this book” with the phrase “distribution of voseo” to find it. It’s under copyright so I can’t reproduce it here.
As you can see in either map, the distribution of voseo doesn’t tidily follow country borders, or even continental borders. The controlling factor is, rather, historical. As I explained in my very first post on this blog, travel between Spain and Latin American was restricted during the colonial period because of rampant piracy on the Atlantic Ocean. Therefore, settlements close to the two colonial capitals of Mexico City and Lima, or to the major ports of call en route such as Veracruz and Portobelo, had much more exposure to the latest linguistic developments from Spain than those in the “boondocks”.
For your convenience, here is the map of colonial trade routes I included in that first post.
Voseo is a perfect illustration of this phenomenon. At the beginning of the Colonial period, tú and vos were both current in Spain. Eventually, of course, tú won, but only those parts of Latin American that were in regular contact with Spain followed its lead. That’s why, if you compare the two maps, the all-tú areas (grey on the voseo map) roughly correspond to the colonial trade routes (red on the second map). Argentina was about as boondock-y as you could get since it could only be reached by crossing the Andes, by foot and/or by mule, from Lima. That’s why its voseo is the strongest in the continent.