I don't think it was 18. I was trained on 18, but when I got back to my new job they were still using 16. I don't remember being trainedwith the model tree and thenhaving to work without it. Seems like that would be painful.
I remember 19 being a buggy release for a lot of folks. We skipped it. 20 was more stable, but introduced the first icons. It was the first release that I remember having a 'pre-production' build. It was the end of the every-six-month releases as well. Now we're lucky to get one every 18 months.