Lithuanian is a Baltic language, closely related to Latvian, although they are not mutually intelligible. It is written in an adapted version of the Roman script. Lithuanian is believed to be the linguistically most conservative living Indo-European tongue, retaining many features of Proto Indo-European. Lithuanian language studies are important for comparative linguistics and for reconstruction of Proto-Indo-European language. Lithuanian was studied by linguists such as Franz Bopp, August Schleicher, Adalbert Bezzenberger, Louis Hjelmslev, Ferdinand de Saussure, Winfred P. Lehmann, Vladimir Toporov and others.