Confused with voltage divider
All voltages are relative to something; if you're not specifying, it's usually ground, which is 'Z' in this schematic.

Thus, 'voltage XY' is the voltage across the 40 kiloohm resistor, which is the difference between the positive supply voltage and the base. 'voltage YZ' is the voltage across the 10 kiloohm resistor, which is the difference between the ground and the base.

Since you're usually talking about voltage relative to ground, the latter is what you want if you just want to know 'the voltage on the base'.