
der Schlitz
slot / slit
A long, narrow opening or cut in a surface, for example in a mailbox, a machine, or a piece of clothing.
Wirf die Münze bitte in den Schlitz des Automaten.
Please drop the coin into the slot of the machine.
slot / fly
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slot / slit
A long, narrow opening or cut in a surface, for example in a mailbox, a machine, or a piece of clothing.
Wirf die Münze bitte in den Schlitz des Automaten.
Please drop the coin into the slot of the machine.

fly
The opening at the front of a pair of trousers that can be closed with buttons or a zipper.
Achtung, dein Schlitz ist offen.
Careful, your fly is open.
The meaning of 'Schlitz' (slot / fly) becomes clear from context. If people are talking about machines, mailboxes, or walls, it means a narrow opening. In connection with trousers ('die Hose'), it means the opening at the front.
Der Brief passt nicht durch den schmalen Schlitz.
The letter doesn't fit through the narrow slot.
In the meaning of 'Öffnung' (opening), 'Schlitz' is often combined with verbs of dropping something in or pushing something through. Typical examples are 'eine Münze in den Schlitz werfen' (to drop a coin into the slot) or 'einen Zettel durch den Schlitz schieben' (to push a note through the slot).
Für ein Ticket müssen Sie drei Münzen in den Schlitz werfen.
For a ticket, you have to drop three coins into the slot.
The discreet notice 'Dein Schlitz ist offen' (your fly is open) is a very common and useful phrase in everyday life. It is used to point out to someone that the zipper on their trousers is open.
Ein Freund hat mich leise darauf hingewiesen: „Du, dein Schlitz ist offen.“
A friend quietly pointed it out to me: 'Hey, your fly is open.'
cut
A Schnitt is generally a separation or opening caused by cutting. A Schlitz, by contrast, is a long, narrow opening that often has a specific function. With fabric, use Schnitt for the resulting cut line and Schlitz for the elongated opening in a garment.

Ein schmaler Schlitz an der Seite macht den Rock bequemer.
A narrow slit on the side makes the skirt more comfortable.
Ein gerader Schnitt trennt den Stoff in zwei Teile.
A straight cut separates the fabric into two pieces.