from the webpage: http://www.mml.cam.ac.uk/call/help/accents.html (Cambridge University, England)

Accents for PCs, Macs, & HTML

PC applications often use their own conventions for reproducing accented characters; check the documentation or online Help facility for details. In many cases, however, it's possible to insert an accented character by means of a code number. First, hold down the ALT key immediately to the left of the spacebar. Next, still holding down the ALT key, go to the numeric keypad to the right of the keyboard, and type 0 followed by the appropriate three-digit code (as shown below). In some situations, having the Num Lock key pressed on or off may make a difference. At first sight, using numeric codes to type accented characters seems cumbersome, but the more you use this method, the easier it becomes: some of the numbers become memorized while others are guessable, and you do not have to learn a different scheme for each application.

For Macs, the process involves specific keystrokes. One way is pressing a number of keys simultaneously (for example holding down the ALT key when you press the c key produces ç). Another way is with a two-stage key press, one combination after another (for example, to get the é character, you first hold down the ALT key and press e, and then, after releasing, press the e key by itself).

In HTML documents, the safest way to give an accented character is by means of its entity reference: for example é produces é. Entity references ensure that accented characters work on different platforms since it's the local software which converts the reference into an accented character. In many cases, however, if you are using internationally recognised standard characters (ISO-8859), they will also work on different types of machine.

 
Character PC
LeftALT+0+		 Mac HTML   Character PC
LeftALT+0+		 Mac HTML
À A grave 192 ALT+` then A À   à a grave 224 ALT+` then a à
Á A acute 193 ALT+e then A Á   á a acute 225 ALT+e then a á
 A Circ 194 ALT+i then A    â a Circ 226 ALT+i then a â
à A tilde 195 ALT+n then A à   ã a tilde 227 ALT+n then a ã
Ä A umlaut 196 ALT+u then Ä   ä a umlaut 228 ALT+u then a ä
Å A ring 197   Å   å a ring 229 ALT+a å
Æ AE ligature 198 ALT+SHIFT+' Æ   æ ae ligature 198 ALT+SHIFT+' æ
Ç C cedilla 199 ALT+SHIFT+C Ç   ç c cedilla 231 ALT+c Ç
È E grave 200 ALT+` then E È   è e grave 232 ALT+` then e è
É E acute 201 ALT+e then E É   é e acute 233 ALT+e then e é
Ê E circonflex 202 ALT+i then E Ê   ê e circonflex 234 ALT+i then e ê
Ë E umlaut 203 ALT+u then E Ë   ë e umlaut 235 ALT+u then e ë
Ì I grave 204 ALT+` then I Ì   ì i grave 236 ALT+` then i ì
Í I acute 205 ALT+e then I Í   í i acute 237 ALT+e then i í
Î I circonflex 206 ALT+i then I Î   î i circonflex 238 ALT+i then i î
Ï I umlaut 207 ALT+u then I Ï   ï i umlaut 239 ALT+u then i ï
Ñ N tilde 209 ALT+n then N Ñ   ñ n tilde 241 ALT+n then n ñ
Ò O grave 210 ALT+` then O Ò   ò o grave 242 ALT+` then o Ò
Ó O acute 211 ALT+e then O Ó   ó o acute 243 ALT+e then o ó
Ô O circonflex 212 ALT+i then O Ô   ô o circonflex 244 ALT+i then o ô
Õ O tilde 213 ALT+n then O Õ   õ o tilde 245 ALT+n then o õ
Ö O umlaut 214 ALT+u then O Ö   ö o umlaut 246 ALT+u then o ö
Ø O slash 216 ALT+SHIFT+O Ø   ø o slash 248 ALT+SHIFT+o ø
Œ OE ligature 156 ALT+SHIFT+Q Œ   œ oe ligature 140 ALT+q œ
Ù U grave 217 ALT+` then U Ù   ù u grave 249 ALT+` then u ù
Ú U acute 218 ALT+e then U Ú   ú u acute 250 ALT+e then u ú
Û U circonflex 219 ALT+i then U Û   û u circonflex 251 ALT+i then u û
Ü U umlaut 220 ALT+u then U Ü   ü u umlaut 252 ALT+u then u ü
  Y acute 221         y acute 221    
            ß eszett ligature 223 ALT+s ß
  Y umlaut 159 ALT+u then Y     ÿ y umlaut 255 ALT+u then y ÿ
¿ Inverted question 191 ALT+SHIFT+? ?   ¡ Inverted exclaim 161 OPTION+1 ¡
« Left angle quote 171 ALT+\ «   » Right angled quote 187 ALT+SHIFT+\ »
€ Euro 128 ALT+2 € or €