Several hypotheses have been proposed concerning the origin of the graph b in the Thai script. These hypotheses are reviewed, and are shown to have some weaknesses. The issue of the origin of b is examined anew. When the ancient Thai people borrowed the Khmer script and adapted it (with remarkable precision) to their own language, they used the graph for the sound /w/; they then needed to create a new graph to transcribe the sound /b/, for which no letter was available in the ancient Khmer system. Our hypothesis is that this b was made by doubling the graph p, the sound /b/ being phonetically sufficiently close to implosive /ɓ/ (written p) to suggest such a derivation.