rechteckig
rectangular
Having the shape of a rectangle; with four straight sides and four right angles.
Der neue Tisch hat eine große, rechteckige Platte aus Holz.
The new table has a large, rectangular wooden top.

rectangular
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rectangular
Having the shape of a rectangle; with four straight sides and four right angles.
Der neue Tisch hat eine große, rechteckige Platte aus Holz.
The new table has a large, rectangular wooden top.

The word is built from three parts: 'recht' (in the sense of 'rechter Winkel' (right angle), 90°), 'Eck' (corner), and the adjective ending '-ig'. So it literally describes something that has 'right corners.'
Ein Blatt Papier ist normalerweise rechteckig.
A sheet of paper is usually rectangular.
The adjective 'rechteckig' (rectangular) is derived directly from the noun 'das Rechteck' (rectangle). If you know the noun, you can easily work out the meaning of the adjective.
Zeichne bitte ein großes Rechteck auf das Papier.
Please draw a large rectangle on the paper.
'Rechteckig' (rectangular) is often used to describe the shape of everyday objects. Typical nouns are: 'ein rechteckiger Tisch' (a rectangular table), 'ein rechteckiges Fenster' (a rectangular window), 'ein rechteckiges Grundstück' (a rectangular plot of land), or 'ein rechteckiger Raum' (a rectangular room).
Unsere Küche ist lang und rechteckig.
Our kitchen is long and rectangular.
round / about
'rund' and 'rechteckig' both describe shapes, but they are clearly opposed. 'rund' means a shape without corners, like a circle or a sphere. 'rechteckig' means a shape with four straight sides and four corners, like a sheet of paper or a table. When talking about the shape of an object, the difference is very practical: without corners it is round; with four straight sides it is rectangular.

Du brauchst für das Regal eine rechteckige Kiste.
You need a rectangular box for the shelf.
Für den Geburtstag kauft Lisa einen runden Kuchen.
Lisa buys a round cake for the birthday.