Question on hip sizing?

When it says measure hips for size charts, is it the hipbone they're talking about or your butt measurement? I thought it was your butt measurement, but I was reading different hip to size forums and lots of people were talking about measuring their hipbone for sizes, like where low rise jeans normally sit. If that's the real way to do that, i'd be aloooot smaller in pants than I thought I was(x especially since my hipbone and butt measurements are like a seven/eight inch difference!