A letter grade is a system used to evaluate and assign a numerical value or grade to represent the level of academic performance, achievement or proficiency in a particular subject. The grades are usually represented by letters such as A, B, C, D, F etc., where each letter corresponds to a specific range of scores. For example, an "A" grade may indicate excellent work and could be assigned for a score ranging from 90-100%, while a "D" grade might represent satisfactory performance but below average, given for a score between 60-70%. The letter grades are commonly used in schools and universities to assess students' progress and provide feedback on their work.