Visual Marginalia (VM) are a common phenomenon in school, but they hold untapped potentials. We aim to understand their nature and role in learning and thinking. Marginalia are described as making annotations in book margins. In this paper, we extend this concept to include visual self-expressions such as para-writings and para-drawings, creative drawings and writings at the margin of books or notebooks, modification of book pictures, titles or texts, squiggling and doodling. VM are part of students' self-regulatory processes but are frowned upon by teachers. Thus, VM are marginal but also marginalized. We present a theoretical discussion of the topic, a new semiotic methodology that maps visual marginalia in a real classroom, and a model that reveals unstudied ways that students self-regulate.