Etymology
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-logy 

word-forming element meaning "a speaking, discourse, treatise, doctrine, theory, science," from Greek -logia (often via French -logie or Medieval Latin -logia), from -log-, combining form of legein "to speak, tell;" thus, "the character or deportment of one who speaks or treats of (a certain subject);" from PIE root *leg- (1) "to collect, gather," with derivatives meaning "to speak (to 'pick out words')." Often via Medieval Latin -logia, French -logie. In philology "love of learning; love of words or discourse," apology, doxology, analogy, trilogy, etc., Greek logos "word, speech, statement, discourse" is directly concerned.

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-logue 
word-forming element meaning "one who is immersed in or driven by," mostly from French-derived words, ultimately from Greek -logos, -logon (see -logy). Now mostly superseded by -loger, -logist except in ideologue and a few others. As a combining element meaning "kind of discourse," it is from French -logue, from Greek -logos.
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-y (4)

suffix indicating state, condition, or quality; also activity or the result of it (as in victory, history, etc.), via Anglo-French and Old French -é, from Latin -ia, Greek -ia, from PIE *-a-, suffix forming abstract or collective nouns. It is etymologically identical with -ia and the second element in -cy, -ery, -logy, etc.

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-ology 

word-forming element indicating "branch of knowledge, science," now the usual form of -logy. Originally used c. 1800 in nonce formations (commonsensology, etc.), it gained legitimacy by influence of the proper formation in geology, mythology, etc., where the -o- is a stem vowel in the previous element.

The second element is prop[erly] -logy (-logue, etc.), the -o- belonging to the preceding element; but the accent makes the apparent element in E[nglish] to be -ology, which is hence often used as an independent word. [Century Dictionary] 
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-ia 
word-forming element in names of countries, diseases, and flowers, from Latin and Greek -ia, noun ending, in Greek especially used in forming abstract nouns (typically of feminine gender); see -a (1). The classical suffix in its usual evolution (via French -ie) comes to Modern English as -y (as in familia/family, also -logy, -graphy). Compare -cy.

In paraphernalia, Mammalia, regalia, etc. it represents Latin or Greek -a (see -a (2)), plural suffix of nouns in -ium (Latin) or -ion (Greek), with formative or euphonic -i-.
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