Take the word "grammar". Some people write "grammer", but these people haven't met the family - the other words that are closely related to "grammar". Meet "grammarian" and "grammatical", both showing the crucial second "a". Admittedly this "a" looks a bit odd when you compare "grammar" with "hammer", "stammer", "programmer", "windjammer" and "crammer"; but then you look at the family background and it makes more sense. The point is that English spelling is only partly phonological - based directly on pronunciation via the grapheme-phoneme correspondences of KS1 and 2.
But English is more demanding because spelling is partly morphological. Words like "grammar" can be divided into smaller parts (morphemes) which are found (with similar meanings) in other words. Thus "gramma" is found in "grammatical" and "grammarian", so we write "a" to remind ourselves of this similarity. If we wrote "grammer" we would lose this link - and might even start to think that a "grammer crammer" was a place which crams pupils so that they can gram.
The morphological role of spelling is even more obvious with other words. Why does "disappear" have just one "s", when "dissuade" has two? Because "disappear" = "dis+appear" whereas "dissuade" = "dis+suade" (think of "persuade"). And what about "disease" and "dissolve"? Same again:
"dis+ease" and "dis+solve" (think "solve" and "solvent"). The ultimate test of spelling prowess is "separate". How does morphology explain the first "a"? Easy - think of "parent" or even "part". When you're born you separate from your parent; and what you separate from the rest is a part; so "separate" = "se+par+ate". Other family members include "pre+pare" and (unfortunately) "re+pair". Can't win 'em all.
Why is this important? Spelling is one area of language that provokes insecurity and nervousness. It's easy for our students to assume that you're born either a good or bad speller. And being told simply to look up a spelling in the dictionary can simply reinforce failure (imagine being asked to look up "cynical").
One of the most useful services we can perform as teachers is to show the links between word families - for example, by constructing visual word-webs. As a result we can help students to build their confidence in recognising the connections between words, making informed guesses about difficult spellings, and developing a proactive approach to spelling, rather than the passive response that dictionaries can provoke.
Even teenagers are likely to find families like these welcome rather than an embarrassment.
Richard Hudson is professor of linguistics at University College, London Geoff Barton is headteacher of King Edward VI School, Bury St Edmunds, Suffolk