This linguistic study is concerned with the role of the emphasizing particle la- (known as lām al-ta'kīd) in the grammatical traditions of Classical Arabic, as well as with the question of the historical relationship connecting this particle to a set of elements in several other Semitic languages showing comparable forms and functions. Although these particles have hitherto seemed to defy a coherent reconstruction, the very complexity of the data to which they attest proves to provide a key to their interpretation. They represent a critical first step in the refining of our understanding of the history of the Semitic sonorant phonemes.