Diacritical mark on Italian Mac keyboard layout

Every keyboard layout contains just a bunch of letters pertaining to the corresponding alphabet that a particular keyboard was made for.

For example, on the Macintosh Italian keyboard there are the most used accented letters in Italian words such as à, è, é, ì, ò, ù.
But from time to time it is necessary to use some other letters such as the Germanic ü.
Without switching the keyboard layout in System Preferences panel, is easy to obtain those letters, using dead keys.

A dead key produces no output when it is pressed, but modifies the output of the next key pressed after it.

Here you can see how is simple to get the most used character in Western country alphabets.

The option key is the key labelled with "alt". These key combinations are intended for Italian keyboard only.

´ option-8 (á, é, í, ó, ú)
` option-9 (à, è, ì, ò, ù)
¨ option-u (ä, ë, ï, ö, ü, ÿ)
^ option-^ (â, ê, î, ô, û)
~ option-n (ã, õ, ñ)

which is then followed by the letter the accent is intended for.
To obtain ñ press option plus the n key, release them, then press the n key again. In Mac OS X, pressing one of these key combinations creates the accent and hi-lights it, then produces the finalized character when a supported character is pressed, otherwise it leaves the accent remaining.

See Wikipedia for an exhaustive explanation.