The language actually spoken by the characters in LotR, and indeed the language the Red Book was originally written in, was called Adûni, which name Tolkien rendered into English as Westron. Tolkien explains: "The language represented in this history by English was the Westron or 'Common Speech' of the West-lands of Middle-earth in the Third Age. In the course of that age it had become the native language of nearly all the speaking-peoples (save the Elves) who dwelt within the bounds of the old kingdoms of Arnor and Gondor; that is along all the coasts from Umbar northward to the Bay of Forochel, and inland as far as the Misty Mountains and the Ephel Dúath. It had also spread north up the Anduin, occupying the lands west of the River and east of the mountains as far as the Gladden Fields. At the time of the War of the Ring at the end of the age these were still its bounds as a native tongue." (Appendix F) While the Westron of Gondor had an archaic flavour, the hobbits spoke a rustic dialect of it. It is further stated that Westron was also used as a second language by all those who still retained a speech of their own, such as the Drúedain (Woses) and the Rohirrim. Even Orcs used a debased form of Westron when it was needed. In Mordor, Frodo and Sam understood what the two Orcs that were trying to sniff them out were saying to one another, for "being of different breeds they used the Common Speech after their fashion" (LotR3/VI ch. 2). Westron is the language to learn before you enter the time-machine and travel back to the Third Age. (Learning Quenya instead would be like learning Latin before going to Europe: There wouldn't be too many people able to understand you when you arrived.)

In origin, Westron was "a Mannish speech, though enriched and softened under Elvish influence. It was in origin the language of those whom the Eldar called the Atani or Edain, 'Fathers of Men', being especially the people of the Three Houses of the Elf-friends who came west into Beleriand in the First Age". In the Second Age, the Adûnaic of Númenor was spoken in the forts and havens that the Númenóreans maintained upon the coast of Middle-earth, "and mingled with many words of the languages of lesser men it became a Common Speech that spread thence along the coasts among all that had dealings with Westernesse" (Appendix F). This process continued after the Downfall: "The people of Elendil were not many, for only a few great ships had escaped the Downfall or survived the tumult of the Seas. They found, it is true, many dwellers upon the westshores who came of their own blood, wholly or in part, being descended from mariners and from wardens of forts and havens that had been set there in days gone by; yet all told the Dúnedain were now only a small folk in the midst of strangers. They used, therefore, the Westron speech in all their dealings with other men, and in the governing of the realms of which they had become the rulers; and this Common Speech became now enlarged, and...much enriched with words drawn from the Adûnaic language of the Dúnedain, and from the Noldorin [read: Sindarin]." (PM:33-34) According to PM:315, Westron changed from the original Adûnaic partially by neglect: The surviving Faithful of Númenor had no great love of Adûnaic, this being the language of the rebel Kings of Westernesse that had tried to suppress all other tongues. Yet the language was later "softened under Elvish influence". Tolkien described Westron as "about as mixed as mod[ern] E[nglish]" (Letters:425). The Elvish elements in Westron can probably be compared to the numerous French words that have become naturalized to English.

THE STRUCTURE OF WESTRON
We know very little about Westron, for the simple reason that Tolkien has rendered it into English almost everywhere! A few words of genuine Westron are given in Appendix F to LotR and (relatively) many more in The Peoples of Middle-earth. Tolkien even translated the names of the Hobbits. There were never any hobbits called Frodo, Sam, Pippin and Merry; their real names were Maura, Ban, Razar and Kali. The word hobbit itself is just a rendering of the actual Third Age word kuduk (derived from Old English holbytla "hole-dweller" the way kuduk is believed to descend from archaic kûd-dûkan of this meaning, the form kûd-dûkan still being preserved in Rohirric). Maura ("Frodo") and his friends would not have known the word "hobbit" as such; they said kuduk.

Concerning the phonology and structure of Westron, David Salo observes (private communication): "The [consonant] sounds of late Adunaic and Westron are almost the same. They have in common p, b, t, d, k, g, m, n, ng, r, ph, th, s, z, h, y, l. Westron is said in LotR to have the palatals ch, sh, but only sh was exemplified in the material. Westron also has hr-, hl-. No w is exemplified in Westron, but Westron has v, which Adunaic did not. Conceivably Westron could have changed w > v. Westron words are not entirely dissimilar from Adunaic: they have what could be triconsonantal root words (gamba 'he-goat', tapuk 'rabbit', galab 'game', laban 'bag', narag 'dwarf', zilib or zilbi 'butter', and a large number of biconsonantals: rama 'cottager', zara 'old', bana "half", rapha 'burr'."

The vowels constitute a classical five-vowel system: short a, e, i, o, u and long â, î, ô, û; long ê is not attested in any actual word, but its existence is implied by a footnote in Appendix E. (It is there stated that some speakers of Westron used ei and ou, "more or less as in English say no", instead of ê, ô - this pronunciation, though "fairly widespread", was held to be incorrect and rustic. Needless to say, this was the usual pronunciation among the Hobbits.) Reportedly, Westron also had certain reduction vowels.

Westron did not possess the Quenya sounds ty, hy; Gondorian speakers of High-elven substituted ch (as in church) and sh. Neither did Westron have ch as in German ach; see UT:319. Therefore, pure Sindarin Rochand, Rochan became Rohan in Gondorian pronunciation.

One late phonological change is mentioned in PM:320: Double (long) consonants were reduced to single ones medially between vowels, tunnas "guard" being pronounced tunas (but not normally so spelt). Consonants in certain combinations were altered; tunnas itself represents earlier tudnas.

Endings
An agental ending -a is seen in such words as pûta "blower", batta "talker". The ending -a was also a masculine ending (PM:46), at least in the Hobbit dialect. Tolkien, when translating the Red Book, Anglicized such names by changing this ending to -o, e.g. "Bilbo" for genuine Hobbitish Bilba. The endings -o and -e were feminine; Tolkien may have substituted -a for -o.

The plural ending seems to be -in, as in cûbuc "hobbit" pl. cûbugin (PM:49 - cûbuc changed to kuduk in the published LotR). Tolkien considered several plural endings before he settled on -in, such as -a, -il, -en. (The idea of unvoiced plosives becoming voiced before this plural ending, as in cûbuc/cûbugin, was apparently dropped later.)

It seems that Westron, like the Scandinavian languages, employs a suffix instead of an independent definite article: Sûza "Shire", Sûzat "The Shire".

The original, archaic Westron seems to have had case endings, but by the end of the Third Age, the endings had been lost. Nargian in Phurunargian "Dwarrowdelf" is a fossilized form of the genitive plural of narag "dwarf". David Salo theorizes: "Since Adunaic has no true genitive, one has to suppose that over the course of the Third Age, Adunaic was transformed (via agglutination of suffixes) into a full-fledged case language, and then subsequently lost case endings again. Nargian could be *nargii (a plural stem, incorporating the old Adunaic ending -i) + an, the old [Adûnaic] 'genitive' marker, now postposed instead of preposed."

The words raza "stranger", razan "foreign" seem to argue the existence of an adjectival ending -n.

The past participle may have the ending -nin; see karnin below.

We know no Westron pronouns, but we know something about them: "The Westron tongue made in the pronouns of the second person (and often also in those of the third) a distinction, independent of number, between 'familiar' and 'deferential' forms. It was, however, one of the peculiarities of Shire-usage that the deferential forms had gone out of colloquial use. They lingered only among the villagers, especially of the West-farthing, who used them as endearments. This was one of the things referred to when people of Gondor spoke of the strangeness of Hobbit-speech. Peregrin Took, for instance, in his first few days in Minas Tirith used the familiar forms to people of all ranks, including the Lord Denethor himself. This may have amused the aged Steward, but it must have astonished his servants. No doubt this free use of the familiar forms helped to spread the popular rumour that Peregrin was a person of very high rank in his own country." (Appendix F) It proved impossible to represent these Westron pronominal distinctions adequately in in Tolkien's English translation of the Red Book.

Elvish Influence
The strong Elvish influence on Westron is seen even in our small corpus. Some of these words may have been borrowed from Avarin by the ancestors of the Edain, passing into Westron via Adûnaic, some may have been borrowed from Sindarin by the Dúnedain exiles after the Downfall.

balc "horrible" seems related to the Sindarin word balch "cruel", derived from the primitive base ÑGWAL "torment" (LR:377).

batta "talker" is undoubtedly related to Primitive Quendian KWET, Common Telerin *PET = "speak"; cf. Sindarin peth "word", lenited beth.

karnin *"cloven" (tentatively isolated from Karningul "Rivendell") seems to be related to the Elvish stem SKAR "tear, rend"; the participal ending -nin is also very similar to the Sindarin ending -nen (as in dirnen, tirnen "guarded, *watched" from tir- "watch"; cf. Talath Dirnen "the Guarded Plain").

nas "people", stated in PM:320 to be borrowed from either Sindarin nos or Quenya nossë, "kindred, family". ("The short o of Elvish became a in such words," evidently because they were borrowed at the Adûnaic stage. Adûnaic had only long ô, but did possess short a; the borrowings altered the quality of the vowel instead of the quantity.)

nîn "water" must be related to the Elvish stem NEN "water", Quenya nén, Sindarin nen pl. nîn.

ras "horn" (musical); cf. Quenya rassë "horn", Sindarin -ras as in Caradhras "Redhorn"

zîr "wise" is very similar to Quenya saira.

Westron/Hobbit wordlist
(all rejected forms excluded; Tolkien experimented much. Where PM forms disagree with LotR forms, the former are usually silently omitted. Tolkien's spelling is retained throughout, but c and k represent the same sound, k being preferred in LotR - see Tûk).

Adûni "Westron" (PM:316)

ba-, ban(a) "half" (PM:51), banakil, "halfling, hobbit" (Appendix F, final notes)

balc "horrible" (UT:313)

Ban "Sam", often held to be short for Bannâtha just like Sam is short for Samuel, but in Sam Gamgee's case his name was short for Banazîr. (PM:51)

Banazîr "half-wise, simple" (Appendix F)

bara- "quick?" Isolated from Barabatta "Quicktalker" (PM:52)

-bas "-wich" (PM:48, Appendix F, final notes)

batta "talker"? Isolated from Barabatta "Quicktalker" (PM:52)

Bilba "Bilbo" (PM:50)

Bophîn "Boffin" (meaning forgotten; the name is simply Anglicized) (Appendix F)

bolg- "bulge" (PM:48)

Bralda-hîm "heady ale", pun of the name of the Baranduin (or Branda-nîn) river, translated "Brandywine" (Appendix F, final notes)

branda- "border, march"; Brandagamba "Marchbuch", Branda-nîn "Border-water", "Marchbourn" (Appendix F, final notes)

Bunga "Bungo", Bunga Labingi "Bungo Baggins" (PM:48)

castar a coin of some sort, of which a tharni was the fourth part (PM:45)

gad- "stay". In Ranugad, q.v.

galap, galab- "game" (PM:48/Appendix F, final notes)

Galbasi "Gamgee" (Appendix F, final notes)

gamba "he-goat", in Brandagamba "Brandybuck" (Appendix F, final notes)

gul "valley?" (tentatively isolated from Karningul "Rivendell") (Appendix F)

hamanullas unidentified small blue flower, tentatively translated "lobelia" (PM:47)

hîm(a) "ale, beer" (PM:54) In Bralda-hîm, q.v. (Appendix F, final notes.)

hloth(o) "cot", two-roomed dwelling (PM:49)

hloth-ram(a) cottager, "cotman" (PM:49). Hlothram "Cotman", the name of Farmer Cotton's grandfather. (Appendix F, final notes)

Hlothran "Cotton"; see Lothran.

kali "merry, jolly, gay"; Kalimac a name of forgotten meaning but inevitably associated with kali; hence Tolkien rendered Kalimac by Meriadoc and the short form Kali by Merry. (Appendix F)

karnin "cloven?" (tentatively isolated from Karningul "Rivendell", Appendix F)

kast "mathom" (from Rohirric kastu; this word was probably used only in the Hobbit dialect of Westron)

kuduk "hobbit", used in the hobbit dialect only; other speakers of Westron used the term banakil "halfling" (Appendix F, final notes)

laban "bag"; Labingi "Baggins" (PM:48); Laban-neg "Bag End" (PM:83)

Lothram "Cotman" (PM:49)

Lothran "Cotton", Hobbit village name (PM:49). It contains of hlotho + rân, q.v. Spelt Hlothran in Appendix F, final notes.

luthur, luthran "down, fluff" (PM:49)

Maura "Frodo" (PM:50) There was no word maur- in contemporary Westron, but in archaic Rohirric it meant "wise, experienced"; hence Tolkien rendered Maura by a Germanic name of similar sense.

narag- "dwarf" (PM:58), archaic gen.pl. nargian as in Phurunargian "Dwarf-delving, Dwarrowdelf". (Appendix F)

nas "people". In tudnas, q.v. Borrowing from Quenya nossë or Sindarin nos, "kindred, family". (PM:320)

neg "end" (noun) (PM:83)

nîn "water". In Branda-nîn, q.v.

Ogmandab "Gorhendad" (an Oldbuck) (PM:83)

phârë "speech"; Sôval Phârë "Common Speech"

phur- "delve"; phûru, "delving" (archaic); Phurunargian "Dwarrowdelf".

pûta "blower" (*pût- "blow"?) In Raspûta, q.v.

rân "a village, a small group of dwellings on a hill side" (PM:49), ran(u) "home, village" ("ham") Ranugad = "Hamfast, Stay-at-Home" (Appendix F)

râph(a) "burr" (noun) (PM:60). In Zilbirâpha.

ras- "horn"; Raspûta "Hornblower" (PM:45, 47)

raza "stranger"; razan "foreign" (PM:51)

Razanur Tûc "Peregrin Took" (PM:51); cf. Razar.

razar a small red apple; Razar "Pippin", associated with the apple-word but actually short for Razanur. (PM:51)

ribadyan "byrding", one celebrating a birthday (Letters:290)

sôval "common"; Sôval Phârë "Common Speech" (PM:55) (Actually we cannot be absolutely certain which part means "common" and which means "speech")

sûza sphere of occupation; division of a realm; Sûza "Shire", Sûzat "the Shire" (PM:45)

tapuc "rabbit, coney" (PM:49)

tarkil "person of Númenórean descent" (Appendix F)

tharantîn "quarter, fourth part" (PM:45)

tharni "quarter, farthing" (the fourth part of a coin, but also used of the farthings of the Shire) (PM:45)

trah- a Hobbit stem that apparently has to do with creeping through a hole; see PM:54.

trân "smial" (probably unique to the Hobbit dialect; cf. Rohirric trahan). (Appendix F)

TUD "watch, guard" (stem) (PM:320)

tudnas "guard" (a body of men acting as guards). Later tunnas, also so spelt; even later pronounced with a short (single) n, but still normally spelt with double nn; the incorrect spelling tunas occurred in the original Book of Mazarbul and was rendered by the equally incorrect spelling gard in Tolkien's reconstruction of this page (that did not make it into the published LotR). See PM:320 and TI:458.

Tûk (so spelt in Appendix F, Tûc in PM:46) "Took" (According to the tradition of the Tooks, tûca "was an old word meaning 'daring', but this appears to be a wholly unfounded guess"; hence, Tolkien simply anglicized the spelling).

zara- "old"; Zaragamba "Oldbuck" (Appendix F, final notes)

zîr(a) "wise"; Banazîr "Half-wise, Samwise" (Appendix F, PM:51)

zilib, zilbi- "butter"; Batti Zilbirâpha "Barney Butterburr" (PM:60, 52)

P.S: In Vinyar Tengwar #32, Carl F. Hostetter and Patrick Wynne argued that whatever the Westron word for garden is, it must begin with a G, just like the English word. This is evident from Galadriel's words to Sam when she gave him a box with a silver rune on the lid before the fellowship left Lórien: "Here is set G for Galadriel, but it may stand for garden in your tongue." Hostetter and Wynne argued that the Westron word for "garden" is ultimately derived from the primitive Elvish stem 3AR (LR:360), which is remarkably similar to the Indo-European stem to which English garden can be traced. "English garden is thus ultimately of Eldarin descent," they conclude. "We can claim that there are indeed 'fairies at the bottom of our garden'."