Novial/AIL Idiom Neutral
The first decades after the birth of Esperanto contributed several books on the question with new "languages," most of them written by former Volapükists, and containing vocabularies based on West-European languages with more or less artificial grammars. None of them had any practical influence, but those of Lott, Liptay and Heintzeler deserve mention here, because they show a growing conciousness, that the task is not so much to invent a language as to find out what is already in international use, and to utilize that to the utmost extent. All the while some members of the Volapük Academy were patiently and steadily at work under the presidency of W. Rosenberger of St. Petersburg, and when finally they brought out their language under the title of Idiom Neutral (1902), it turned out to be something as different from Volapük as day is from night. In the vocabulary, the maximum of internationality had been aimed at, this being calculated by counting how many of the seven principal languages contained a particular word. The result was a language that could be read with comparative ease by every educated person. On the whole, we have here a very conscientious piece of work, which has deeply influenced all subsequent schemes, though as a system it gained very few adherents, probably because its grammar was inadequate in various points, and because too little had been done to avoid disturbing homophones; moreover its many consonant-groups (e.g. nostr patr "our Father") made the language far from euphonious.
Among numerous systems of the same type, but not worked out to the same extent as Neutral, I shall here mention only H. Molenaar's Universal (1906).