Here, here.

Curiously, the commodities markets have always adjusted tick sizes for different contracts so that the dollar value is reasonably consistent (~10 dollars). These are products for which the contract size (in dollars) varies wildly and the quotes are all over the place (anywhere from 9 decimal points to no decimal points).

It makes writing your algos confusing, esp when you start doing inter-commodity spreads, but the business case is so obvious, it really makes you wonder why the equities markets haven't done more.