vocabulary - I need a word to describe the quality of a rectangle

I would like a more formal word that describes the rectangle on the left as opposed to the slanted one on the right. For some reason, I can't seem to think of a description without being wordy

How Long Does it take to Write 300 Words?, by Katie Mitchell

What is Rectangle? Definition, Examples, Formula, Facts

Second Grade Dolch Sight Word Flashcards

150 Great Complimentary Words and Phrases to Use in Customer Service

The end of 'trump,' or how to wreck a perfectly good word

Bird Word Wall Vocabulary

Za' and 9 Other Words to Help You Win at SCRABBLE

Audio) 3000 Most Common Words in English – Meaning and Sentence - Basic English Speaking

The Heart Word Method for Teaching Sight Words - FREE Template