Employing non-full-rank design matrices throughout, this text provides a concise yet solid foundation for understanding basic linear models. It introduces the basic algebra and geometry of the linear least squares problem, before delving into estimability and the Gauss-Markov model. After presenting the statistical tools of hypothesis tests and confidence intervals, the author analyzes mixed models, such as two-way mixed ANOVA, and the multivariate linear model. The text presents proofs and discussions from both algebraic and geometric viewpoints and inclu*es exercises of varying levels of difficulty at the end of each chapter.