pharmacy, photograph, etc.
Why and what determines the use of f or ph in the spelling of a word?
English spelling does more than just spell units of sound. It also spells units of meaning and tries to show the historical sources of words that we have borrowed from other languages. That English tries to spell all three – sounds and meanings and sources – helps makes English spelling as complicated as it is. Whether the sound [f] is spelled <f> or <ph> is pretty much a matter of historical source – with a bit of mathematics thrown in.
For instance, the <ph> in pharmacy comes from its Greek historical source, which was spelled with the initial Greek letter phi, which was transliterated into Latin and some other languages as <ph>. In some languages pharmacy was respelled with an initial <f> to make it more consistent with the languages’ spelling systems. For instance, in Spanish and Italian it is still spelled with an initial <f>. English borrowed the word from Old French, in which it was spelled with an initial <f>, which is how it was spelled in English in the 14th and 15th centuries. But then in the 16th and 17th centuries, a time during which a number of English words were respelled to show their classical Greek and Latin sources, it began to be spelled with an initial <ph> as it was in Latin, and as it still is today.
The digraph <ph> spells [f] in words most often borrowed from Greek – and a few from Hebrew. Most often, <ph> spells the sound [f] in word-initial position: pharmacy, phase, pheasant, phlegm, phobia, phone, photo, phrase, physical. It often clusters with <s>: asphalt, asphyxiate, sphere, sphinx. In the middle of words it is often in element initial position: aphasia [a+phas+ia], cellophane cell+o+phane, diphthong [di+phthong, emphatic [em+phat+ic], microphone micr+o+phone, prophecy [pro+phec+y]. And finally, <ph>often spells [f] in the middle of elements: alphabet, catastrophe, decipher, gopher, nephew, orphan, pamphlet, typhoon, or at the end: telegraph, triumph, trophy, typhoid. In short, the <ph>spelling can occur most anywhere in a word. In this respect it is like the much more common <f>spelling, which can also show up in any part of the word – though single <f> very rarely spells [f] in word-final position, the only known instances being the native if and the French borrowings chef and clef. Word-final [f] is usually spelled <ff>: staff, stiff, stuff.
Since the [f] sound’s position in the word doesn’t help much in determining whether to spell it <ph> or <f>, history and math are the best clues. The spelling <ph> is most common in Greek words, which are usually quite technical and often have distinctive Greek look, with <y> spelling long or short <i> and <ps>spelling [s], as in psychology. Actually a very few bases account for the great majority of words with the <ph> spelling: phot+, phil(e)+, phys+, phone+, phob(e)+, soph+, sphere, and graph – as in photo, philosophy, physics, telephone, acrophobia, sophomore, atmosphere, telegraph.
The spelling <f> occurs in native English words and in those borrowed from languages other than Greek. But the strongest help probably comes from math: The sound [f] is spelled <ph> only about 12.7% of the time. It is spelled <f> more than six times as often, 79.7%. So when in doubt, go with <f>.
(To get examples of more words with the three spellings of [f], you can go to the CommonWords database on this site and in the drop-down menu for the Correspondences field pick “[f]=<f>”, “[f]=<ff>” or “[f]=<ph>.”)