Letter name knowledge in the preschool ages is a strong predictor of later reading ability, but little is known about the psychometric characteristics of uppercase and lowercase letters considered together. Data from 1,113 preschoolers from diverse backgrounds on both uppercase and lowercase letter name knowledge were analyzed using Item Response Theory. Results indicated that uppercase and lowercase form a single dimension. Uppercase letters tended to be easier and more discriminating but had a narrow range of difficulty. Visual confusability (e.g., b vs. d) was an important aspect of both discrimination and difficulty. Including lowercase letters in the assessment of letter name knowledge increases its range to enable effective measurement of children with higher ability. A practical implication is that assessments of letter name knowledge can have fewer items and measure an extended range of ability while maintaining high levels of reliability.