Overview
A joint account is a bank account that has been opened by two or more individuals or entities. Joint accounts are commonly opened by close relatives (such as by a married couple) or by business partners in an unincorporated business, but it can be used in other circumstances.
Ordinarily, anyone can deposit funds into a joint account, but when opening an account the joint account holders may indicate to the financial institution whether a single account holder may make withdrawals or whether the consent of other account holders is required.
A joint account is not the same as adding an authorized signatory or additional cardholder to an account, that is, a person who is authorized by the account holder to effect transactions on the account. Under this arrangement the primary account holder remains fully and solely liable for all transactions on the account. Accounts by corporate entities are not, in themselves, joint accounts.
Opening an account
When opening a joint bank account, the account holders need instruct the financial institution how and by whom the account is to be operated. They would decide the signatories on the account. For example, withdrawals may require any account holder to sign a withdrawal or all parties to sign the withdrawal or “any two account holders” to sign, or a particular account holder with any other account holder, or some other instructions.
From Wikipedia (CC BY-SA 4.0).