The mapping and recording of small-scale phonological changes poses difficulties, especially as the practical technology of sound recording dates only from the 19th century. Written texts provide the main (indirect) evidence of how language sounds have changed over the centuries. But note Ferdinand de Saussure's work on postulating the existence and disappearance of laryngeals in Proto-Indo-European as an example of other methods of detecting/reconstructing sound-changes within historical linguistics. Poetic devices such as rhyme and rhythm may provide clues to previous phonological habits.