While the letters C, Q, W, and Z are found on the Icelandic keyboard, they are rarely used in Icelandic; they are used in some proper names of Icelanders, mainly family names (family names are the exception in Iceland). The letter C is used on road signs (to indicate city centre) according to European regulation, and cm is used for the centimetre according to the international SI system (while it may be written out as sentimetri). Many believe these letters should be included in the alphabet, as its purpose is a tool to collate (sort into the correct order). The alphabet as taught in schools up to about 1980 has these 36 letters (and computers still order this way): a, á, b, c, d, ð, e, é, f, g, h, i, í, j, k, l, m, n, o, ó, p, q, r, s, t, u, ú, v, w, x, y, ý, z, þ, æ, ö.