I got stuck on whether I should say I'm a frequent flier or flyer. I came across an article on writingexplained.com and it confirmed pretty much what I suspected, that there's no consensus on the flyer/flier preference. Interestingly, it mentioned the AP Stylebook changed its spelling for one who flies from "flier" to "flyer" in 2017.
In its 2017 style book, however, it completely reversed this stance, citing that most airlines don’t follow the practice outlined by AP.
So even more exasperating is that after learning one way, we never know when public opinion or a recommendation will swing, as it did only last year with at least one style guide.
Because every time I write something I don't want to spend ten minutes reading debates and histories on orthography, I wanted to come up my own simple mnemonic or rule. It seems to me that multisyllabic verbs ending in y have an agentive suffix of -ier added to the -y. This is pretty consistent and can't see any departures as yet.
List of words ending in -y listed in order of number of letters
This is all good. I started thinking of short single-syllable verbs. We can leave out these class of words where a vowel precedes the final -y, because these seem to invariably end with -yer:
That leaves only a small handful of English verbs that may cause trouble in their spelling:
cry (NGrams attests overwhelmingly to "crier" over "cryer")
fry (the more common variant is "fryer" over "frier")
NGrams supports this.
NGrams doesn't help as comparative adjective "drier" confounds results.
pry (Dictionaries accept both)
American Heritage Dictionary and Collins English Dictionary
(NGrams shows more results for "prier" than "pryer")
ply (I would expect 'a plier of his trade')
In this case only American Heritage Dictionary lists "plyer" as a variant of "plier"
American Heritage Dictionary
(NGrams shows more results for "plier" than "plyer")
try (seems to be overwhelmingly "trier" according to NGrams)
spy (NGrams overwhelmingly supports "spier" over "spyer")
scry (NGrams shows results for "scryer", but not "scrier", arrgh, just when I see a pattern, it's broken)
These are all the ones I could find, there may be more, but as I said, there are only a handful that cause problems for me at least.
To sum up, I'd say most fall under the -ier category, with three prominent ones falling into the -yer category, these are:
dry = dryer
fry = fryer
scry = scryer
Flyer/flier is hotly contested when referring to someone who flies. An NGram search for flyer would show results for the pamphlet also, so that's a problem. However frequent flyer outnumbers frequent flier by more than double today, both in AmE and BrE. Also, many aeroplanes, boats and trains (including two American trains used circa 1999-2003) were named "flyer", not "flier", with exception of USS Flier submarine launched 1943. So probably "fly" should be added to the above exception list. Merriam-Webster says however that "flier" is more common than "flyer". There seems to be a general consensus for "flyer" to mean the leaflet/pamphlet.
The dryer/drier advice given here suggests to use "dryer" for something/someone that dries, and "drier" as a comparative adjective for "dry". I checked to see whether this follows a general rule of forming the comparative adjective. As far as I can see it doesn't. "Slyer" and "slier" are both given in dictionaries, as also are "shyer" and "shier", with the -yer form being more common according to NGrams. This advice seems pretty arbitrary to me.
If anyone knows advice they can give, I'd appreciate it. If not, hopefully this can be of use for some people. Writing this out has definitely helped me a little in organising these words into categories in my own head.
Please correct anything I've got wrong, I likely have.