The M-Dash and N-Dash
Before the M-dash and the N-dash, there were only the dash and the hyphen.
The hyphen. The hyphen joins two words to make another larger word. The larger word is called a "compound" word because it is made up of more than one part. The hyphen is a key on or near the top right of your keyboard.
The dash. The dash was a mark of punctuation (two hyphens together) that joins two parts of a sentence - not joining two words. The dash acts like an "interruption."
It used to be that you would create a dash with two hyphens together with no spaces between.
Within the past decade or so, the dash became known as the M-Dash (because it is longer) and the hyphen became known as the N-Dash (because it is shorter.)
The M-Dash examples
The N-Dash examples
When a compound adjective appears before the noun, it requires the hyphen (also known as the N-Dash). Take the example above:
If that same phrase appears AFTER the noun, it does not require the hyphen (also called the N-Dash). See below:
On the keyboard
On the keyboard, the key that shows the hyphen (plus the underscore) is the following:
As you may know, if you hold down the Shift Key, the symbol on top of the key will print. In this case, it is the underscore.
That underscore appears at the bottom of the printed line and can be used to create a blank line - particularly useful, if you are preparing a fill-in form.
The hyphen is created by using that highlighted key without the Shift Key - simply on its own.
If you want to create a simply hyphen, then do not leave spaces before or after the hyphen and you will have the popular N-Dash.
With the computer, you can create a dash by typing the following keystrokes:
When the computer sees that combination above, it will "autocorrect" your keystrokes into the popular M-Dash.
If you are handwriting the two symbols, make the dash (the M-Dash) a little longer than the hyphen (The N-Dash).
Make it clear that you are joining two parts of the sentence with the M-Dash and joining more than one word to make another longer word with the N-Dash.
I hope this clarifies your idea of the M-Dash and the N-Dash.