A blueline is a photographic proof, traditionally printed in blue ink, made for checking the arrangement of text, images, margins, and page order before a press run. Because it is chiefly a layout proof, it allows editors and printers to catch misplaced stanzas, incorrect folios, or broken lineation. For a poetry book, reviewing a blueline can be especially important because a single unwanted line break or shifted indentation may alter a poem's intended pacing and visual structure.

Understanding the "blueline" (also called an ozalid or diazo proof) provides crucial context for bibliography, textual criticism, and the history of book production. Historically, the blueline represented the final chance for an author or publisher to catch visual errors, layout misalignments, or missing line numbers before lithographic plates were made. In poetry publishing, layout precise to the millimeter is vital—especially for concrete poetry or stanzaic symmetry—making the blueline phase critical for preserving the poet's intended typography and white space. Related terms include galley proof, signature, imposition, and recto/verso.