waagerecht
horizontal
Aligned parallel to the horizon or to a calm water surface; horizontal. The opposite of 'senkrecht' (vertical).
Bitte ziehe mit dem Lineal eine exakt waagerechte Linie.
Please use the ruler to draw an exactly horizontal line.

horizontal
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horizontal
Aligned parallel to the horizon or to a calm water surface; horizontal. The opposite of 'senkrecht' (vertical).
Bitte ziehe mit dem Lineal eine exakt waagerechte Linie.
Please use the ruler to draw an exactly horizontal line.

The word is derived from 'die Waage' (a scale for weighing). Imagine an old balance scale: when it is balanced, the beam is perfectly horizontal. This helps you remember the meaning 'horizontal'.
Der Horizont ist die beste Referenz für eine waagerechte Linie.
The horizon is the best reference for a horizontal line.
'Waagerecht' (horizontal) can be used as an adjective before a noun or as an adverb to describe an action. It can also stand alone with 'sein' (to be).
Halte das Bild bitte genau waagerecht.
Please hold the picture exactly horizontal.
Common nouns that are combined with 'waagerecht' (horizontal) are 'Linie' (line), 'Position' (position), 'Lage' (position/posture), or 'Fläche' (surface).
Der Patient musste nach der Operation mehrere Stunden in waagerechter Lage bleiben.
After the operation, the patient had to remain in a horizontal position for several hours.
just / exactly
'eben' and 'waagerecht' can easily be confused because, in some contexts, 'eben' may make you think of a straight or exact position. In the uses that matter here, however, 'eben' does not mean 'horizontal'. It describes a very short time, emphasizes something, or expresses that you accept a situation. 'waagerecht', on the other hand, describes the direction or position of something, for example a line or a board. If it is about time, emphasis, or 'halt' (just / simply accepting a situation), 'eben' fits. If something lies parallel to the horizon, 'waagerecht' is more precise.

Für das Regal muss das Brett ganz waagerecht liegen.
For the shelf, the board has to be completely horizontal.
Ich habe eben mit dem Kunden telefoniert.
I just spoke with the customer on the phone.