Why SI symbols follow strict capitalization: the prefix mega is a capital M (lowercase m means milli, one thousandth), and the unit hertz is written Hz with a capital H because it honors Heinrich Hertz, followed by a lowercase z. Putting those together gives MHz, meaning one million cycles per second.
Watch out The version with a lowercase m would read millihertz, one thousandth of a hertz, which is a vastly different quantity, and the forms with a lowercase h or capital Z are simply not valid SI notation.
Big M for million, capital H for Hertz the man: MHz.