At GrammarBook.com you can learn when and when not to use dashes, including the en dash and the em dash. In addition, you'll discover the en dash is about the width of the letter n, and is slightly longer than a hyphen.
By comparison the em dash is the width of the letter m.
A visit to GrammarBook.com will demystify when and why to use one or the other.
Using your PCs keyboard to form an en dash, type the first number or word, then hold down the ALT key while typing 0150 on the numerical pad on the right side of the keyboard. Then type the second number or word.
Using your PCs keyboard to form an em dash, type the first word, then hold down the ALT key while typing 0151 on the numerical pad on the right side of the keyboard. Then type the second word. You can also form an em dash by typing the first word, hitting the hyphen key twice, and then typing the second word. Your word processor program will turn the two hyphens into an em dash.
Also, be sure to visit BLOGGERdotCOM where you'll find Cheatsheet For Bloggers .