"-er" vs "-re"

     [This is a fast-access FAQ excerpt.]
The following words are spelled with "-re" in the U.K. but with
"-er" in the U.S.:  accoutre(ment), calibre, centre, fibre, goitre,
litre, louvre, lustre (brilliance, but "luster" one who lusts) ,
manoeuvre ("maneuver" in the U.S.), metre (for the distance and
for poetic and musical metre, but "meter" for the measuring device),
meagre, mitre, nitre, ochre, philtre, reconnoitre, sabre, sceptre,
sepulchre, sombre, spectre, (amphi)theatre, titre.  (The British
"metre"/"meter" distinction is retained when the various prefixes
are prepended:  "kilometre", "speedometer", etc.  "Micrometer", a
device for measuring minute things, is distinguished from
"micrometre", a micron.  "Theatre" has some currency in the U.S.,
especially in names of specific theatres.)
   The following words are spelled "-re" in both the U.K. and the
U.S.:  acre, cadre, euchre, lucre, massacre, mediocre, ogre,
wiseacre.  (The "-cre" and "-gre" words may have been kept that
way in order to keep the "c" and "g" hard, although there are
counterexamples such as "eager" and "meager".)
   In none of these words is "-er" the agent suffix (as in
"revolver") or the comparative suffix (as in "longer").  Most of
these words come from Latin through French, and they took the "-re"
form in French because the "e" was not part of the word root.  (The
adjectives tend to be in "-ral", "-ric", and "-rical", rather than
"-eral", "-eric", or "-erical".)  But many similar words
(cloister, diameter, neuter, number, sinister) were changed from
"-re" to "-er" in English.  The process has merely happened faster
in the U.S. than in Britain.