When a security is brought before book-closure date i.e the cut off date where all the members registered in the register get dividend which was proposed earlier by the BOD..The buyer gets dividend rights also and the security is said to be cum-dividend.. In such a situation the acquisition cost must be arrived after deducting the amount recievable in respect of dividends.. For e.g A purchases 500 shares at Rs. 50 cum-dividend and gets Rs. 5 per share dividend.. So cost of acquisition is Rs.500*(50-5).. This can also happen when BOD declares dividend for the period. But you don't have shares of that company during that period..The only reason you got the right to dividend is because your name appears on the register..
suppose u purchase shares of xyz ltd in april 2011....and the company declared dividend on those shares related to the f.y 2010-11....that dividend will be received by u coz your name is on the register...this is pre accquisition dividend which will be deducted from the cost of the investment..
The pre-acquision dividend will reduce the cost of investment It means dividend received for a period in which you did not hold those shares.
Hope that clears.