Calder uses his set of pentominoes throughout the book for a variety of different purposes. Their messages and meanings change depending on his need. The pentominoes themselves are geometric shapes that correspond to different letters of the alphabet, but Calder forms new patterns from those basic shapes. He can spell words with them, fit them together like puzzle pieces to create rectangles, or create an alpha-numeric code to communicate with Tommy. Calder even uses them as a form of divination when he draws a random pentomino out of his pocket to see if the letter provides an answer to whatever problem he’s currently facing.

The chief value of the pentominoes is their flexibility. Because they can be adapted to so many different uses, they accelerate Calder’s ability to perceive alphabetical and numeric patterns. He thinks in terms of numbers, letters, and shapes. More importantly, he sees how each of those components fits with the others.

The motif of letters appears in the very first pages of Chasing Vermeer when three people receive the same anonymous letter. Unbeknownst to the reader, Ms. Hussey is one of the recipients. Perhaps assuming she can trace the writer to someone in Hyde Park, Ms. Hussey tasks her class with finding someone who has received an interesting letter.