In the USA, all tartans are plaid, but not all plaids are tartan. In almost every tartan, the warp and the weft are the same (so your tartan is the same vertical and horizontal). Many plaid fabrics do not have the same warp and weft.

Take a look here for a bunch of plaid fabrics, most of which would not be mistaken for any tartan:
http://www.thefabricfinder.com/PlaidFabrics.htm

Here's one telling of the history of your flannel shirt:
http://www.ehow.com/facts_5022110_hi...el-shirts.html