According to Marchand (1969), clippings are not coined as words belonging to the standard vocabulary of a language. They originate as terms of a special group like schools, army, police, the medical profession, etc., in the intimacy of a milieu where a hint is sufficient to indicate the whole. For example, exam(ination), math(ematics), and lab(oratory) originated in school slang; spec(ulation) and tick(et = credit) in stock-exchange slang; and vet(eran) and cap(tain) in army slang. Clipped forms can pass into common usage when they are widely useful, becoming part of standard English, which most speakers would agree has happened with math/maths, lab, exam, phone (from telephone), fridge (from refrigerator), and various others. When their usefulness is limited to narrower contexts, such as with tick in stock-exchange slang, they remain outside standard register. Many, such as mani and pedi for manicure and pedicure or mic/mike for microphone, occupy a middle ground in which their appropriate register is a subjective judgment, but succeeding decades tend to see them become more widely used.
Clipping is different from back-formation - back-formation may change the part of speech or the word's meaning, whereas clipping creates shortened words from longer words, but does not change the part of speech or the meaning of the word.
Final (or back) clipping is the most common type, in which the beginning of the prototype is retained. The unclipped original may be either a simple or a composite. Examples are: ad (advertisement), cable (cablegram), doc (doctor), exam (examination), fax (facsimile), gas (gasoline), gym (gymnastics, gymnasium), memo (memorandum), mutt (muttonhead), pub (public house), pop (popular music).
Initial (or fore) clipping retains the final part of the prototype. Examples: bot (robot), chute (parachute), roach (cockroach), coon (raccoon), gator (alligator), phone (telephone), pike (turnpike), varsity (university), net (Internet).
Final and initial clipping may be combined and result in curtailed words with the middle part of the prototype retained, which is the stressed syllable. Examples: flu (influenza), frig or fridge (refrigerator), jams or jammies (pajamas/pyjamas), polly (apollinaris), shrink (head-shrinker), tec (detective).
Words with the middle part of the word left out are equally few. They may be further subdivided into two groups: (a) words with a final-clipped stem retaining the functional morpheme: maths (mathematics), specs (spectacles); (b) contractions due to a gradual process of elision under the influence of rhythm and context. Thus, fancy (fantasy), ma'am (madam), and fo'c'sle may be regarded as accelerated forms.
Clipped forms are also used in compounds. One part of the original compound most often remains intact. Examples are: cablegram (cable telegram), op art (optical art), org-man (organization man), linocut (linoleum cut). Sometimes both halves of a compound are clipped as in navicert (navigation certificate). In these cases it is difficult to know whether the resultant formation should be treated as a clipping or as a blend, for the border between the two types is not always clear. According to Bauer (1983), the easiest way to draw the distinction is to say that those forms which retain compound stress are clipped compounds, whereas those that take simple word stress are not. By this criterion bodbiz, Chicom, Comsymp, Intelsat, midcult, pro-am, photo op, sci-fi, and sitcom are all compounds made of clippings.