The American Heritage® Dictionary of the English Language: Fourth Edition. 2000.
Appendix I
Indo-European Roots
ENTRY:
ar-
DEFINITION:
Also ar-. To fit together. Oldest form *2er1-, colored to *2ar1-, with variant *2re1-, contracted to *r-. Derivatives include army, harmony, inert, aristocracy, adorn, hatred, rite, arithmetic, and rhyme. I. Basic form *ar-.1. Suffixed form *ar()-mo-.a.arm1, from Old English earm, arm, from Germanic *armaz;b.ambry, arm2, armada, armadillo, armature, armoire, army; alarm, disarm, gendarme, from Latin arma, tools, arms; c.armillary sphere, from Latin armus, upper arm. 2. Suffixed form *ar()-smo-.harmony, from Greek harmos, joint, shoulder. 3. Suffixed form *ar()-ti-.a.art1, artisan, artist; inert, inertia, from Latin ars (stem art-), art, skill, craft; b. further suffixed form *ar()-ti-o-.artiodactyl, from Greek artios, fitting, even. 4. Suffixed form *ar()-tu-.article, from Latin artus, joint. 5. Suffixed form *ar()-to-.coarctate, from Latin artus, tight. 6. Suffixed form *ar()-dhro-. arthro-; anarthrous, diarthrosis, dysarthria, enarthrosis, synarthrosis, from Greek arthron, joint. 7. Suffixed (superlative) form *ar()-isto-.aristocracy, from Greek aristos, best. II. Possibly suffixed lengthened o-grade form (or separate root) *r-dh-.1.ordain, order, ordinal, ordinance, ordinary, ordinate, ordo; coordination, inordinate, subordinate, from Latin rd, order (originally a row of threads in a loom). 2.exordium, primordial, from Latin rdr, to begin to weave. 3.ornament, ornate; adorn, suborn, from Latin rnre, to adorn. III. Variant *r- (< *re-). 1.rate1, ratio, ration, reason; arraign, from Latin rr, to consider, confirm, ratify. 2. Suffixed form *r-dh-.a. (i)read, rede; dread, from Old English rdan, to advise; (ii)hatred, kindred, from Old English rden, -rden, condition. Both (i) and (ii) from Germanic *rdan;b. (i)rathskeller, from Old High German rt, counsel; (ii)riddle2, from Old English rdels(e), opinion, riddle. Both (i) and (ii) from Germanic *rdaz.3. Zero-grade form *r-. Germanic *radam, number, in dialectal North and West Germanic compound *hund(a)-rada- (see dek). IV. Variant root form *2re1i-, with zero-grades *2r1i- and (metathesized) *2ri1-, the latter contracted to *r-.1. Suffixed metathesized zero-grade form *r-tu-.rite, from Latin rtus, rite, custom, usage. 2. Suffixed unmetathesized zero-grade form *ri-dhmo-.arithmetic, logarithm, from Greek arithmos, number, amount. 3.rhyme, from a Germanic source akin to Old High German rm, number, series. (Pokorny 1. ar- 55.)