OK, I hope I don't sound too confusing, but I've always wondered the difference between this symbol '--' and the parentheses '()' in terms of grammar in writing. That sword (which was a priceless one, mind you) was the one that just went over the cliff. That sword -- which was a priceless one, mind you -- was the one that just went over the cliff. Basically, how do they work? Are they interchangeable? If they make the sentence different in their own usage, how so? Here's another example, this time through the thought of a character. I looked up in time to see Pierce's helicopter (where did he get one?) fly into view. I looked up in time to see Pierce's helicopter -- where did he get one? -- fly into view. I'm curious if there are any differences in the sentence structure. Thoughts?