The maize leaf is composed of distinct regions with clear morphological boundaries. The ligule and auricle mark the boundary between distal blade and proximal sheath and are amenable to genetic study due to the array of mutants that affect their formation without severely affecting viability. Herein, we describe the novel maize gene extended auricle1 (eta1), which is essential for proper formation of the blade/sheath boundary. Homozygous eta1 individuals have a wavy overgrowth of auricle tissue and the blade/sheath boundary is diffuse. Double-mutant combinations of eta1 with genes in the knox and liguleless pathways result in synergistic and, in some cases, dosage-dependent interactions. While the phenotype of eta1 mutant individuals resembles that of dominant knox overexpression phenotypes, eta1 mutant leaves do not ectopically express knox genes. In addition, eta1 interacts synergistically with lg1 and lg2, but does not directly affect the transcription of either gene in leaf primordia. We present evidence based on genetic and molecular analyses that eta1 provides a downstream link between the knox and liguleless pathways.