What is the more appropriate term for this idiom, up or down?
Right ____ your alley.
According to this idiom site, either are in use, but I would like to get the opinion of this erudite crowd.
Does the answer depend on the slope of the alley?
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While down your alley appears to have been somewhat popular in the 1930s and 1940s, its usage has dropped considerably, and up your alley is vastly more popular in modern usage (by about a factor of seven). However, both phrases have seen recent usage, according to the samples linked at the bottom of the Ngram page.
Anecdotally (as a native Mideast U.S. speaker, born well after the 1940s), I cannot recall ever hearing down your alley used idiomatically, but I am very familiar with up your alley as an idiom.
This usage nearly always features up, not down. The AmE version is usually alley...
...but the BrE version is usually street...
The meaning is the same in all cases - something is exactly what you're interested or skilled in. It's irrelevant to ask about "the slope of the alley", since it's a figurative usage anyway (no "real" alley or street is being referenced).
The only version I've ever used personally, is: "Right up my alley," although if someone were to say it "Right down my alley," I would know what they meant, but it does sound a little odd; another saying which is very close in meaning to this one, is: "That's just what the doctor ordered," suggesting that the subject being discussed, is something which would be approved by one's physician, (metaphorically, of course).