der Besitz
possessions
The totality of things that belong to a person; property or assets.
Nach dem Brand war ihr gesamter Besitz vernichtet.
After the fire, all her possessions had been destroyed.

possession
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possessions
The totality of things that belong to a person; property or assets.
Nach dem Brand war ihr gesamter Besitz vernichtet.
After the fire, all her possessions had been destroyed.

The noun 'der Besitz' (possession/property) is closely related to the verb 'besitzen' (to own/possess) and the person 'der Besitzer' (the owner). If you know one of these words, you can understand and use the others more easily.
Er besitzt mehrere Immobilien in der Stadt.
He owns several properties in the city.
The phrase 'etwas in Besitz nehmen' (to take possession of something) is very common and means to take control of something. The noun after 'nehmen' is always in the accusative.
Nach der Renovierung konnte die Familie ihr neues Haus endlich in Besitz nehmen.
After the renovation, the family was finally able to take possession of their new house.
When referring to a person's assets or property, 'Besitz' is often combined with adjectives such as 'privat' (private), 'fest' (fixed/real), or 'gesamter' (entire). A common phrase is also 'in den Besitz von etwas gelangen' (to come into possession of something).
Durch die Erbschaft gelangte er in den Besitz eines wertvollen Gemäldes.
Through the inheritance, he came into possession of a valuable painting.
In a legal context, 'Besitz' describes actual control over an object, which is not always the same as legal ownership. A tenant, for example, has an apartment in his possession.
Der Finder des Schmucks hatte ihn zwar im Besitz, war aber nicht der Eigentümer.
The finder of the jewelry had it in his possession, but was not the owner.
thing / matter
'die Sache' and 'der Besitz' can both have to do with things that someone has. However, 'Sache' is much more general and everyday. In the plural, 'Sachen' often means personal things such as clothing, a bag, or keys. 'Besitz' more often means everything that belongs to someone, often taken together as property or assets. The word sounds more formal and is also used in legal contexts. Learners may confuse the words because both talk about things. As a simple guide: for individual or personal things, people usually say 'Sachen'; for property as a whole, 'Besitz' is more likely.

Nach dem Umzug hat Max seinen ganzen Besitz neu geordnet.
After the move, Max reorganized all his possessions.
Im Zug passt Frau Becker gut auf ihre Sachen auf.
On the train, Ms. Becker keeps a close eye on her things.