For a couple last seasons horizontal stripes has been kind of a trend. In addition to vertical stripes horizontal stripes have been seen in shirts, knitwear and even in tailored clothing. Last summer I bought myself a double-breasted jersey blazer by Tombolini. A jersey blazer that features thin blue and white horizontal stripes. In fact it is a great summer sport coat for casual occasions. I don't usually recommend wearing striped jackets as sport coats. For example chalk striped suit jacket should generally be used only as a part of full suit. However this rule is not carved in stone. A bold striped sport coat can be a great statement piece. And if you choose a jacket with horizontal stripes - that is quite another thing. As you can't really have a suit with horizontal stripes. Restrained combination of brown and blue.