The rejection region is the area in $n$-dimensional space of observed values that would lead us to reject the null hypothesis, where $n$ is the number of observations. Formally, for $X_1, X_2, \dots, X_n$ the size of the rejection region is given by the joint probability $P(X_1 \in R_1, X_2 \in R_2, \dots, X_n \in R_n) = P(\vec X \in R; \theta_0)$ For a test based on a single observation $X_1$, the rejection region is a segment of a number line that contains the values of $x_1$ that would lead us to reject the null hypothesis. For a test based on two observations $X_1$ and $X_2$, the rejection region will be defined by a line bisecting the $X_1$, $X_2$ space. For example, if the form of the test is to reject the null hypothesis if $\bar X > 5$, the region above and to the right of the coordinates for $(X_1, X_2)$ of $(10, 0)$ and $(0,10)$ is the rejection region. #diagram