Among constructed languages, it is usual to distinguish between a priori languages and a posteri languages. The latter are based on one or, more often, several source languages, while this is not evident for a priori languages, e.g., the philosophical languages of the 17th century, Solresol and the logical languages of the 20th century, such as Loglan and Lojban. Spontaneously arisen Interlanguages are necessarily a posteriori or iconic (using imaging or imitating signs).