When does it become 'your' pattern?

[tawg.livejournal.com]

A while back there was a really interesting discussion here about selling embroidery pieces and the legality of using someone else's pattern, and it got me thinking.

I've recently taken up knitting again, and I've been making little toy mice from a pattern that I got out of a magazine (the magazine in turn got it from the book 'Knitted Toy Tales' by Laura Long). I've made a few tiny changes to the pattern, and I was wondering when you feel that a pattern becomes 'yours', if ever?