The first hour of every parliamentary sitting is slotted for this.
During this time, the members ask questions and the ministers usually give answers.
The questions are of three kinds, namely, starred, unstarred and short notice.
 A starred question requires an oral answer and hence supplementary questions can follow.
 An unstarred question, on the other hand, requires a written answer and hence, supplementary questions cannot follow.
 A short notice question is one that is asked by giving a notice of less than ten days. It is answered orally.
It is an informal device available to the members of the Parliament to raise matters without any prior notice.
The zero hour starts immediately after the question hour and lasts until the agenda for the day (i. e, regular business of the House) is taken up.
In other words, the time gap between the question hour and the agenda is known as zero hour.
It is an Indian innovation in the field of parliamentary procedures and has been in existence since 1962.