A letter of instruction (also called a testamentary letter or side letter) is an informal, nonlegal document that generally accompanies your will and is used to express your personal thoughts and directions regarding what is in the will (or about other things, such as your burial wishes or where to locate other documents). This can be the most helpful document you leave for your family members and your executor.
Unlike your will, a letter of instruction remains private. Therefore, it is an opportunity to say things you would rather not make public.
A letter of instruction is not a substitute for a will. Any directions you include in the letter are only suggestions and are not binding. The people to whom you address the letter may follow or disregard any instructions.