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Between 1932 and 1939 the creators of Betty Boop, Fleischer Studios, created 90 films. The ownership of some of the films changed hands and a number of the Betty Boop films failed to re-register the copyright (U.S. copyright law was different in the 1930’s than it is today). Some of the Betty Boop films fell into the Public Domain, while others retained their copyright. Nimia licenses eight Betty Boop clips for commercial use and Internet Archives hosts 22 films that are all in the public domain.

.... there you go!!!

copyrights or author rights remain for 80 year max to the original creator according to international law..... so if you write a book at age 20, and manage to reach the age of 101, the day you turn 101 you loose the royalty's for it. As far I understand the international law says about it