I'm reading Thermal Physics by Kettle and I feel I'm having trouble really grasping what entropy is. From my lower division pretty much entropy can be defined as a measurement of randomness in a system. Like with ice, very little randomness=lower entropy, water, more randomness because the molecules can move around=higher entropy. Kettle brings up conventional entropy and fundamental entropy. I'm having trouble really understanding the difference and why do we have both. Conventional entropy seems to be the one you learn in lower division courses, it has units. It's the fundamental entropy multiplied by the Boltzmann's constant. The book states the fundamental entropy is a pure number. So why have both, and how does one benefit over the in certain problems? Thank you.