Can't find it: Scheduliing subjects using three columns

Last week, I stumbled across either a blog article or a discussion post where Sonya (I believe) was explaining how to break down each subject into three columns, for example:

Sitting       reading     listening

Sitting       drawing    listening

Or something to that effect and then scheduling the day’s work in a way that no two subjects involved the same three criteria. Sadly, I did not bookmark the spot and now I can’t find it for love or money. 🙂  Can anyone point me in the right direction?