Words can be included in Interlingua in either of two ways: through regular derivation using roots and affixes or by establishing their eligibility as international words. The second of these methods is explained below.
The theory underlying Interlingua posits an international vocabulary, a large number of words and affixes that are present in a wide range of languages. Social forces, most notably the dynamism of science and technology, have spread this vocabulary to „all corners of the world“. The goal of the International Auxiliary Language Association was to accept into Interlingua every widely international word in whatever languages it occurred. For practical reasons, however, IALA’s researchers could not examine all the world’s languages. Therefore, they conducted studies to identify a small group of languages that would deliver „the most generally international vocabulary possible“, while still maintaining the unity of the language.
The languages selected are called control languages. The primary controls are English, French, Italian, Spanish, and Portuguese, with Spanish and Portuguese taken as one language. The secondary controls are German and Russian. According to the rule of three, a word is eligible for Interlingua if it occurs in at least three of the four primary control languages, with either or both of the secondary control languages acting as possible substitutes.
To provide generally international words, the control languages had to have a high degree of radiating power and a high degree of receptive power. In particular, they had to „radiate“ a large number of words into other languages and to „absorb“ a large number from other languages. Thus, IALA developed the concepts of productive and receptive spheres of language.
In classical antiquity, Latin and Greek were the languages of the dominant political and cultural forces. Afterward, these languages acted for two millennia as essential lingua francas in Western science and religion. As a result, the Western languages have imported many thousands of Greek and Latin words, either through ancestry or through transfer and loan. Many of these same words are found in non-Western languages, such as Arabic, Hindi, Swahili, and Japanese.
The IALA considered Greek and Latin, together with their descendents the Romance languages, to be the primary components of the productive sphere. English was the chief representative of the receptive sphere. In modern times, however, English has become a prolific source of international words, „radiating“ them to such varied languages as Japanese, Arabic, Hindi, and Urdu, as well as to most Romance, Germanic, and Slavic languages.
Initially, this view of an Anglo-Romance productive sphere permitted the researchers of IALA to greatly simplify the derivation of the international vocabulary. They could omit research into the receptive sphere, because all of the words that the languages of this sphere held in common were taken from the productive sphere. For example, Dutch and Hungarian have little in common, being of different language families, except for variations of many thousands of Greco-Latin international words such as information, international, and politics. These words are already found in the productive sphere. Thus, the addition of Dutch and Hungarian to Interlingua’s control languages would not affect the resulting vocabulary.
As research progressed, IALA found that a close adherence to the Anglo-Romance productive sphere led to the omission of some clearly international words. As a result, the rule of three was adjusted. The two most prominent languages in the receptive sphere, German and Russian, became control languages. A word that was missing from one or two of the productive controls was then eligible for Interlingua if it occurred in one or both, respectively, of the added receptive controls.
The Iberian languages Spanish and Portuguese were considered as one language not because their separate importance was in question, but because their linguistic roles in the modern world were similar.
Because the international vocabulary has a substantial Greek component, the absence of Greek as a control language sometimes evokes surprise. The paths by which Greek words enter the international vocabulary are different from those of the control languages. Greek contributed large numbers of international words in ancient times, leading to a large body of Greek vocabulary in Vulgar Latin. This vocabulary has accompanied Latin language material into the international vocabulary. In addition, many scientific and technical terms that make use of the Greek are coined in such languages as German and English. Many of these words, such as automobile and telephone, have passed into common use. Through these routes, Greek words become part of Interlingua without the Greek language itself being used as a control.