subsidary company

 As per Companies act -a company , be deemed to be a subsidiary of another if,
(a) that other controls the composition of its Board of directors; or
b)
(i)  exercises or controls more than half of the total voting power of such company;

(ii) where the first-mentioned company is any other company, holds more than half in nominal value of its equity share capital; or
(c) the first-mentioned company is a subsidiary of any company which is that other's subsidiary.

Illustration
Company B is a subsidiary of Company A, and Company C is a subsidiary of Company B. Company C is a subsidiary of Company A, by virtue of clause (c) above. If Company D is a subsidiary of Company C, Company D will be a subsidiary of Company B and consequently also of Company A, by virtue of clause (c) above; and so on.