The spi n quantum number ms

The fourth and final quantum number is the spin quantum number ms. This one describes the direction the electron is spinning in a magnetic field — either clockwise or counterclockwise.

Only two values are allowed for ms: +1⁄2 or –1⁄2. For each subshell, there can be only two electrons, one with a spin of +1⁄2 and another with a spin of –1⁄2.