die Rückseite
back / reverse side
The rear or opposite-facing side of an object, as opposed to the front side.
Die Adresse steht auf der Rückseite des Briefumschlags.
The address is on the back of the envelope.

back / reverse side
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back / reverse side
The rear or opposite-facing side of an object, as opposed to the front side.
Die Adresse steht auf der Rückseite des Briefumschlags.
The address is on the back of the envelope.

'Rückseite' (back / reverse side) is a compound noun made up of 'Rück-' (from 'zurück', meaning back) and 'Seite' (side). So it literally describes the rear or opposite-facing side of an object.
Die Notizen stehen auf der Rückseite des Papiers.
The notes are on the back of the paper.
To describe the location, you almost always use the preposition 'auf' with the dative: 'auf der Rückseite' (on the back). This is especially true for flat objects like paper, cards, or photos.
Auf der Rückseite der Postkarte steht eine kurze Nachricht.
There is a short message on the back of the postcard.
The word 'Rückseite' (back / reverse side) is typically used for things that have two sides. For example, people talk about the back of a sheet of paper, a coin, a building, or a medal.
Auf der Rückseite der Medaille ist das Datum eingraviert.
The date is engraved on the back of the medal.
front side
'Rückseite' and 'Vorderseite' refer to two opposite sides of an object. The 'Vorderseite' is the side that faces forward or that you see first, for example on a sheet of paper, a card, or a building. The 'Rückseite' is the back or opposite-facing side. You can confuse the words if you are not sure which side counts as the front. A simple tip: the most important information or the picture often goes on the 'Vorderseite'; additional details are often found on the 'Rückseite'.

Auf der Rückseite der Einladung steht die genaue Adresse.
The exact address is on the back of the invitation.
Herr Richter schreibt seinen Namen auf die Vorderseite des Formulars.
Mr. Richter writes his name on the front side of the form.