behalf of Tibet. Sikkim–Tibet boundary definition Called the Convention of Calcutta, the Anglo-Chinese treaty of 1890 recognised British suzerainty over Sikkim and delineated the boundary between Sikkim and Tibet. The border was defined as the watershed between Teesta River of Sikkim and Mochu of Tibet (on the Dongkya range), starting at "Mount Gipmochi". For today's point of view, what was meant by "Mount Gipmochi" is unclear as no land surveys of the area had been undertaken prior to the treaty. Travel maps and sketch maps available from that time period show no awareness of the Doklam plateau on the part of the British, placing Mount Gipmochi directly on the Dongkya range. (See Map 3.) Continued Tibetan resistance to the acceptance of the Anglo-Chinese Treaty eventually led to a British expedition to Tibet in 1904, led by political officer Francis Younghusband. The ensuing Convention of Lhasa obtained the Tibetan agreement to the earlier terms. The boundary established between Sikkim and Tibet in the treaty still survives today, according to scholar John Prescott. Exploration of Doklam In the course of the Younghusband expedition, Charles Bell was asked to lead a team investigating a supply route to Tibet through Bhutan, via the Amo Chu valley. The team travelled through Sipchu and Sangbay, and then, along an existing "goat track" on a mountain ridge, went up to the Doklam plateau. After reaching Mount Gipmochi, they descended to the Chumbi Valley through Sinchela. This appears to have been the first instance of British exploration of the Doklam plateau. Through this exploration, Bell also discovered that the prevailing border between Bhutan and Tibet to the west of Amo Chu was a highland tree (Ya-shing) – lowland tree (Mön-shing) border over the same geographic region. The highland trees belonged to Tibet, perhaps above 11,500 ft in elevation, while the lowland trees belonged to Bhutan. With the geography of the Doklam plateau becoming known, the later maps show a divergence. The official Survey of India map shows the correct location of Mount Gipmochi on the border of India (but without showing the borders of Bhutan) as per treaty. (Map 5) Unofficial maps often show Batang La peak—the peak corresponding to Gipmochi on the Dongkya Range—as the trijunction of the three countries. (Map 4 and 6.) Relations between Bhutan and India Bhutan became a protected state (though not a 'protectorate') of British India in 1910, an arrangement that was