We started to get our girls trimmed shorter (still have long coat just trimmed down a bit) for the summer about a year ago. Angel turned 3 in December and her coat is so thick that sometimes it is hard to get the comb through it. It now grows so long that it touches the floor and she has actually tripped on it. She was 9 months old when we got her from the rescue and she had never been groomed. Her coat was thin but shiny. I had no idea how thick her coat would become over the years. Our groomer sits down with us before she does any grooming and we go over specifics.
I am hoping that her Sheltie has grown its hair back by now 
Despite my efforts my family thinks shelties only come in three colors: sable, tri, and blue merle. I can't get them to understand what bi black and bi blue are. "It's a TRI. Like Fluffy!" No, Mom. It's a bi black. Fluffy has tan markings. Tri = three. Bi black are black and white. Like a skunk.
(Harharhar. Don't get mad at the skunk comment.)
