One of the oldest continuous Malay monarchies, traditionally dated to the 1136 conversion of its ruler to Islam — heir to the far older Bujang Valley polity of Old Kedah. Kedah controlled the fertile northwestern plain of the peninsula, exported rice and tin, and spent centuries balancing tribute to Siam against Achehnese and Bugis pressure: Penang was ceded to the East India Company in 1786, Siam occupied the state outright in 1821-1842 (the sultan in exile in Penang), and the 1909 Anglo-Siamese Treaty finally drew the modern border — Kedah to Britain, Setul (Satun) and Songkhla staying Siamese.
Non-terminal: British protectorate from 1909, unfederated Malay state, and since 1957 a constituent monarchy of modern Malaysia.
The atlas holds 3 border frames for Kedah Sultanate — the shape you scrub through changes in 1136 · 1786 · 1909.
Standard scholarly survey covering Kedah's tributary position between Siam and Aceh.