"tombadas" could also apply to the cut of the fabric, as i think it's the case; but if it's just talking about the direction of the stitches, then it would be "slanted seam".

"All clothes' seams are slanted and double stitched"
"All clothes are bias-cut and double-stitched" I think this is the case.

Some extra info that might help you out:
http://www.globefiresuits.com/cairns/products/reaxtion/trous...
the waistband is cut on the bias (diagonally) for stretch and an unrestricted fit.

http://www.singer.com.br/eu_que_fiz/dicas_ideias/descricao.a...
2. Passe as costuras, pelo direito do tecido, tombando-as para o mesmo lado. Cada uma das fileiras deve ter as costuras tombadas para um lado diferente

http://66.102.9.104/search?q=cache:210qOkSNOgQJ:www2.dgpatr....
Todas as costuras deverão ser tombadas para o meio das costas ou para o meio da frente, sendo no caso das mangas, para trás

http://vintagesewing.info/1920s/26-fcm/fcm-14.html
http://sewing.about.com/library/sewnews/library/aafash19.htm
Avoid bulky dirndls and tiered skirts, and bias-cut skirts that cling to curves.

:-))