When you own things jointly
It is important to understand jointly owned possessions as they can be held in two very different ways. A jointly owned house, for instance, can be held as a "joint tenancy" which means that when one of the owners dies their share will pass automatically to the survivor, whatever the will says. If it is held as a "tenancy in common" the share of the first to die passes according to their will or, if there is no will, the rules of intestacy.
The deeds of a flat will show whether the owners are joint tenants or tenants in common, but if the joint property is a bank account it may not be clear what sort of joint ownership was intended. If you share a bank account or property with someone else you need to decide what is to happen if one of you dies. Does the dead partner's share go to the survivor or does it go with the rest of their estate? How is their share calculated? You need to put it into writing, sign the agreement and each keep a copy.
If it seems complicated, take legal advice before you sign the agreement. Whatever the agreement you reach, you should make sure that the deeds or details of the bank account match your agreement.