Till 1947, Sikkim was an Indian princely state ruled by Chogyal. In 1947, after the lapse of British paramount, Sikkim became a ‘protectorate’ of India, whereby the Indian Government assumed responsibility for the defense, external affairs and communications of Sikkim.  

• In 1974, Sikkim expressed its desire for greater association with India. Accordingly, the 35th Constitutional Amendment Act (1974) was enacted by the parliament.  
• This amendment introduced a new class of statehood under the constitution by conferring on Sikkim the status of an ‘associate state’ of the Indian Union.  
• For this purpose, a new Article 2A and a new schedule (Tenth Schedule containing the terms and conditions of association) were inserted in the Constitution.  
• This experiment, however, did not last long as it could not fully satisfy the aspirations of the people of Sikkim.  
• In a referendum held in 1975, they voted for the abolition of the institution of Chogyal and Sikkim becoming an integral part of India.  

• Consequently, the 36th Constitutional Amendment Act (1975) was enacted to make Sikkim a full-fledged state of the Indian Union (the 22nd state).  
• This amendment amended the First and the Fourth Schedules to the Constitution and added a new Article 371-F to provide for certain special provisions with respect to the administration of Sikkim.  
• It also repealed Article 2A and the Tenth Schedule that were added by the 35th Amendment Act of 1974. 

Source: Laxmikanth