Define the mutual will and list the requirements which must be met to make its doctrine operate

ANSWER

A mutual will is an arrangement between two people usually a husband and wife to execute mutual wills making provision for each other with the remainder to a third party usually a child.
There must be an agreement to the effect that the survivor cannot revoke the arrangement.
On the death of either party leaving his or her will unrevoked the arrangement becomes binding.
It is an equitable doctrine developed to prevent an unconscionable revocation of a will.