In policy debate, the negative block refers to the second negative constructive (2NC) and the first negative rebuttal (1NR). Although the two speeches are divided by a three-minute cross-examination of the 2NC, they are given back to back without the interruption of an affirmative speech. This is a result of the affirmative having the first and last speech.
Almost universally, negative teams will "split the block" by dividing the arguments between their speeches to avoid repeating themselves. Usually, the division will be based on flows, but sometimes based on second affirmative constructive (2AC) arguments if there is a more compelling reason to divide arguments on flows. Often the 2NC and 1NR will go for different "worlds" of arguments, enabling the 2NR to go for only 2NC or only 1NR arguments, if the opportunity presents itself.
Because the 1NR has the ability to answer arguments which were dropped by the 2NC, the cross-examination of the 2NC will generally not emphasize dropped arguments. Also, because the cross-examination provides de facto preparation time to the 1NR, some debaters will end the cross-examination early if they have no important questions to ask.