The Constitution of Pakistan is said to be rigid and flexible at the same time when it comes to making amendments. A constitution is rigid in the sense that it cannot be amended in the manner in which ordinary laws are passed or amended. In Pakistan, a 2/3rd majority is required to pass a constitutional amendment Bill as compared to a simple majority needed for passing ordinary or institutional Billls. However it is still considered flexible as compared to the US constitution that has a more rigid amendment procedure.
Articles 238 and 239 (in Part XI) provide for the amendment procedure in the 1973 Constitution of Pakistan.
According to Article 238, the Constitution may be amended by an Act of Parliament, which means that it would follow a similar procedure as that of passing a Bill.
According to Article 239, an amending Bill can be initiated in either of the two Houses of Parliament.
For example, it may originate in the National Assembly and after being passed by 2/3rd majority of the House, it shall be transmitted to the other House, the Senate.
If the other House also passes the amending Bill with 2/3rd majority of the total membership, without any further amendments, the Bill shall be presented to the President for assent.
If the Bill originates in one House and is passed accordingly, and then sent to the other House for approval, and if it is also passed in the other House by 2/3rd majority, but with further amendments, it shall be sent back to the House where it originated, for reconsideration.
After reconsideration of the proposed changes by the originating House, if the Bill is passed including those amendments and changes, by 2/3rd majority, then it shall be presented to the President for assent.
If there is an amending Bill relating to the altering of provincial boundaries, then it shall not be presented to the President unless the same Bill has been passed in the concerned Provincial Assembly by the votes of not less than 2/3rd of its total membership.
Article 239 also entails that no amendment of the Constitution shall be called in question in any court on any ground. It should also be noted that according to this Article, there is no limitation on the power of the Parliament to amend any provisions of the Constitution.