das Revers
lapel
The folded-back, upper front part of a suit jacket, coat, or jacket.
Er trug eine kleine Blume am Revers seines Anzugs.
He wore a small flower on the lapel of his suit.

lapel / written undertaking
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lapel
The folded-back, upper front part of a suit jacket, coat, or jacket.
Er trug eine kleine Blume am Revers seines Anzugs.
He wore a small flower on the lapel of his suit.

'Revers' is a loanword from French. This explains the spelling and pronunciation, which are unusual for German words.
Das Revers dieses Mantels ist aus Samt gefertigt.
The lapel of this coat is made of velvet.
You can easily recognize the meaning from the context. If people are talking about clothing such as 'Sakko' (suit jacket), 'Anzug' (suit), or 'Mantel' (coat), it means the lapel. If the topic is legal or diplomatic documents and verbs like 'unterschreiben' (to sign), the rare meaning of a written undertaking is meant.
Er steckte sich eine Nelke ans Revers.
He pinned a carnation to his lapel.
In its main meaning, 'Revers' (lapel) is often used with the preposition 'an' or 'am' (an dem). Typical adjectives are 'breit' (wide) or 'schmal' (narrow).
Die Mode der 70er Jahre war bekannt für breite Revers.
The fashion of the 1970s was known for wide lapels.
The meaning as a written obligation is very rare and belongs to elevated official or legal language. In normal everyday life, you will hardly ever encounter this meaning.
In dem historischen Dokument fand sich ein Revers des Adeligen.
A written undertaking by the nobleman was found in the historical document.
collar
'der Kragen' and 'das Revers' both belong to clothing in the neck and chest area. 'Kragen' means the part of a shirt, coat, or jacket that sits directly at the neck or frames it. 'Revers' means the folded-back upper front part of a jacket, coat, or especially a suit jacket. The words can be confused because both are at the top of a piece of clothing. As a practical rule: the 'Kragen' is at the neck; the 'Revers' lies at the front of a suit jacket.

An seinem Revers steckt eine kleine Blume.
There is a small flower pinned to his lapel.
Der Kragen meines Hemdes ist zu eng.
The collar of my shirt is too tight.