Rotation in Three Dimensions - 3D Math Primer for Graphics and Game Development - page 230

Before we define the terms heading, pitch, and bank precisely, let us
briefly review the coordinate space conventions we use in this topic. We
use a left-handed system, where +x is to the right, +y is up, and +z is
forward. (Check out Figure 1.15 on page 19 for an illustration.) Also, if you
have forgotten how positive rotation is defined according to the left-hand
rule, you might want to flip back to Figure 1.14 on page 17 to refresh your
memory.
Given heading, pitch, and bank angles, we can determine the orientation
described by these Euler angles using a simple four-step process.
Step 1. Begin in the “identity” orientation—that is, with the object-space
axes aligned with the upright axes.
Figure 8.4
Step 1: An object in its identity orientation
Step 2. Perform the heading rotation, rotating about the y-axis, as shown
in Figure 8.5. Positive rotation rotates to the right (clockwise when
viewed from above).
Figure 8.5
Step 2: Heading is the first rotation and rotates
about the vertical axis