This is the biggest mistake you're making when trying to dress business casual

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Contrary to what many men think, there is a measurable difference between a suit jacket and a blazer.

And that difference could make or break your business casual outfit.

Whether or not you can take that suit jacket and wear it as a blazer is almost entirely a case-by-case basis.

Before we jump into some specific rules and things to look for, first I want to make you aware of the easiest way to tell:

If you look at the suit jacket away from the trousers and think that it looks like it's missing its partner, that's a dead giveaway you can't wear it as a blazer.

The goal is to be able to look at a guy wearing a blazer, and not be able to tell if that jacket is supposed to belong to a pair of dress pants.

If you're still not sure, here are some other things to look at:

• Color. Light gray and navy are very safe, and will probably work. Black and charcoal are a little riskier. Keep this in mind. 
• Pattern. Most patterned suit jackets won't work as a blazer. The good news is that if you've got a large enough suit collection to have a patterned one, you probably don't care if you can wear it as a blazer or not. Pinstripes and window panes are both no-nos, but a thicker fabric might offset this.
• Fabric. A fabric like tweed or other "classic" fabric, however, will nearly always work in a blazer context, according to Real Men Real Style. The thicker and less shiny the fabric, the easier it would be to pull off.
• Details. Though this is no longer quite the case, the difference between blazers and suit jackets was all in the detailing like buttons and stitching. Those have largely gone away, but new ones to watch, like if there are peak or notch lapels, should be paid attention to. (Peak lapels mean it probably won't work as a blazer).
• Fit. Blazers should fit slightly looser than suit jackets. A good rule of thumb is if you can wear a sweater underneath it, it can be worn as a blazer, according to GQ.

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