
der Sitz
headquarters
The main location or headquarters of a company, organization, or public authority.
Der Sitz des Unternehmens befindet sich in Frankfurt am Main.
The company's headquarters are located in Frankfurt am Main.
seat / headquarters
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headquarters
The main location or headquarters of a company, organization, or public authority.
Der Sitz des Unternehmens befindet sich in Frankfurt am Main.
The company's headquarters are located in Frankfurt am Main.

seat
A place to sit, for example a chair, armchair, or place in a vehicle.
Im Bus waren leider keine freien Sitze mehr verfügbar.
Unfortunately, there were no free seats left on the bus.

seat
A mandate or position in a parliament, body, or committee.
Die Partei konnte bei der Wahl drei Sitze im Parlament dazugewinnen.
The party was able to gain three seats in parliament in the election.

fit
The way a piece of clothing sits on the body; the fit.
Die Hose hat einen perfekten Sitz und ist sehr bequem.
The pants have a perfect fit and are very comfortable.

seat
The posture of a person sitting on something, especially on a horse.
Die Reitlehrerin lobte ihren festen Sitz im Sattel.
The riding instructor praised her firm seat in the saddle.
'Der Sitz' (seat/headquarters) is the noun related to the verb 'sitzen' (to sit). Many meanings are connected with the idea of sitting or of something or someone having a fixed place.
Nehmen Sie bitte Platz, hier ist noch ein freier Sitz.
Please take a seat; there is still a free seat here.
To describe the main location of a company, the fixed structure '[Firma] hat ihren Sitz in [Ort]' ([company] has its headquarters in [place]) is used.
Die Europäische Zentralbank hat ihren Sitz in Frankfurt am Main.
The European Central Bank has its headquarters in Frankfurt am Main.
In a political context, 'Sitz' (seat) is often combined with verbs such as 'gewinnen' (to win), 'verlieren' (to lose), 'verteidigen' (to defend), or 'erringen' (to gain) when referring to mandates in a parliament.
Die Oppositionspartei konnte bei der letzten Wahl zehn Sitze dazugewinnen.
The opposition party was able to gain ten seats in the last election.
To describe the fit of clothing, the fixed expression 'einen guten/schlechten/perfekten Sitz haben' (to have a good/bad/perfect fit) is used.
Dieses Sakko hat einen perfekten Sitz.
This jacket has a perfect fit.
When talking about posture while riding, people often speak of a 'fester' (firm) or 'sicherer Sitz' (secure seat) in the saddle.
Ein guter Reiter hat einen ruhigen und festen Sitz.
A good rider has a calm and firm seat.
place / space
Der Platz and der Sitz can both refer to a designated place to sit. Platz tends to emphasize the assigned position, while Sitz more strongly refers to the actual seat. On a ticket, Platz is usually the better choice; for a chair or armchair, Sitz is more natural. However, the two words can overlap.

Der Sitz am Fenster ist noch frei.
The window seat is still free.
Auf meiner Fahrkarte ist Platz 12 im Zug reserviert.
My train ticket reserves seat 12.