der Reiter
rider
A person who sits on the back of a horse or another riding animal and rides.
Der erfahrene Reiter hatte sein Pferd perfekt unter Kontrolle.
The experienced rider had his horse perfectly under control.

rider / tab
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rider
A person who sits on the back of a horse or another riding animal and rides.
Der erfahrene Reiter hatte sein Pferd perfekt unter Kontrolle.
The experienced rider had his horse perfectly under control.

'Der Reiter' (rider / tab) is derived directly from the verb 'reiten' (to ride). The ending '-er' here refers to the person performing the action. So a rider is someone who rides.
Schon als Kind lernte sie das Reiten von einem erfahrenen Reiter.
Even as a child, she learned riding from an experienced rider.
You can easily recognize the meaning from the context. If people are talking about horses, animals, or sports, the person is meant. If it is about offices, files, folders, or index cards, the small marker is meant.
Der Reiter galoppierte über das Feld.
The rider galloped across the field.
When describing the position of a rider, German uses the preposition 'auf' with the dative. The rider sits 'auf dem Pferd' (on the horse).
Auf dem alten Foto sitzt ein Reiter stolz auf seinem Schimmel.
In the old photo, a rider sits proudly on his white horse.
The meaning 'marker' is a metaphor. The small slip of paper 'rides', so to speak, on the file or index card, just as a rider sits on a horse. This image helps you remember the second meaning.
Verwende bitte gelbe Reiter für alle abgeschlossenen Projekte.
Please use yellow tabs for all completed projects.
knight
'der Ritter' and 'der Reiter' sound similar, and both can have to do with horses. Even so, they do not mean the same thing. A 'Ritter' is a historical figure from the Middle Ages, an armored warrior with a particular status. A 'Reiter' is simply a person who rides a horse or another animal. Every knight can also be a rider in many situations, but not every rider is a knight. So pay attention to the context: the Middle Ages and fighting point to 'Ritter', while ordinary riding points to 'Reiter'.

Auf dem Feldweg kam uns ein Reiter auf einem braunen Pferd entgegen.
On the dirt road, a rider on a brown horse came toward us.
In der Geschichte kämpft der Ritter für seinen Herrn.
In the story, the knight fights for his lord.