Klassen’s palette is quiet, his weathered backdrops are elegant, and his comic timing is precisely synched to Barnett’s deadpan prose. Triangle fools Square, and the story fools readers, too, as they wait for Square to put Triangle in his place, or for the two to reconcile. Instead, Triangle seems to win this round, even if he does finish the book trapped in his own home. Whereas the humor in Sam and Dave Dig a Hole was subtle and sly, this shape showdown is pure, antic buffoonery.
—Publishers Weekly (starred review)

Barnett and Klassen, whose previous collaborations—Extra Yarn (2012) and Sam and Dave Dig a Hole (2014)—were Caldecott Honor Books, have created a marvelously clever picture book...The simple sentences and repetition are perfect for a young audience, who will be raptly interested in the devious dynamic between the friends. Judging by this offbeat first volume, Barnett and Klassen’s planned trilogy is shaping up to be an excellent one.
—Booklist (starred review)

Klassen's minimalist visuals make for beautiful, surreal landscapes as the shapes go back and forth; Barnett's even-more-minimalist narrative leaves gaps of many shapes and sizes for readers to ponder. Children will be intrigued by the fairy-tale quality of this narrative and may enjoy debating the motivations of its peculiar characters.
—Kirkus Reviews

Cheeky Triangle and ingenuous Square's quirky relationship is reminiscent of Arnold Lobel's Frog and Toad, but with a twist. Klassen does remarkable things with a minimal canvas — shapes and eyes are all he's got, after all...This weird and wonderful picture book presents a whole new angle on shapes —and friendships.
—Shelf Awareness for Readers

Both the occasionally repetitive text and the images make this title a good match for emerging readers. The characters convey an appropriate level of shifty expression through the movement of their eyes, and the ambiguous ending will elicit plenty of opinions from young audiences. An understated ode to mischief that’s sure to please fans of Sam and Dave Dig a Hole.
—School Library Journal

Fans of this author-illustrator team, and of each creator individually, will recognize elements such as Klassen’s trademark simple shapes, sumptuous textures, and expressive eyes, not to mention a pesky antihero and a chase scene that goes in one direction, then back...The appended dedication and biography page includes visual confirmation that the story’s events were all in good fun.
—The Horn Book

There's a lesson about friendship along with an amusing geometry lesson in this charming picture book from the writer-artist collaborators who created the acclaimed "Sam and Dave Dig a Hole" and "Extra Yarn."
—Buffalo News

This is funny stuff and, as to be expected from Mac Barnett and Jon Klassen, delightfully off-kilter.
—BookPage

It’s all in the execution: the perfect text, the visually arresting and funny art (those eyeballs of Klassen’s! He really (re)invented eyeballs in picture books) — it’s a huge read-aloud winner of a book.
—PW ShelfTalker (blog)

Deadpan humor and artistic precision—a combination we’ve come to expect from the wicked Barnett/Klassen collaboration—make this story a joy to read night after night.
—B&N Kids Blog

In the start to another amusing trilogy from the dynamic children’s book duo Mac Barnett and Jon Klassen, some very sneaky shapes are up to no good.
—Pregnancy & Newborn