Will provisions do not control the distribution of assets that are held in Joint Tenancy.

Assets, such as your home, stock and bank accounts,  held in Joint tenancy will be owned by the person or persons who are Joint Tenants with you. So if you are in a second marriage or have a "significant other" relationship and have a Will that gives a portion or all of some assets to your children by the previous marriage, they wont get anything if your new spouse or significant other are listed as Joint tenants. In fact, your Will could say that you are disinheriting the new spouse or anyone else, and it wont  make any difference because Joint  Tenancy trumps your
Will!

The same rule applies to beneficiaries of your IRA and/or 401K and life insurance: The beneficiary designation trumps your Will when there's a conflict.

Be very careful in adding other people's names to bank accounts, stock accounts and real estate without first checking with your attorney. What you assume may not be true with respect to gifting of those assets during life and at death.

Jerry