I love lime leaves. In fact, I bought a lime tree so that I can have them all the time without making a grocery run. The leaves look like this:
I have some very nice recipes that call for them but even my French/Cambodian cookbook with the amazing illustrated ingredient encyclopedia does not clear up one thing:
Is that three leaves or six?
I'm in the middle of making a paste right now that calls for five "lime leaves" and so, according to my usual habit, I'm going to go and pluck 5 leaves off the tree… Which means, I'll have 10 leaf segments. Or will I have 10 leaves?
The taste is so nice that I've never had the feeling I was doubling the amount called for. Still, I'm curious—is there some widespread agreement, or even better, an authoritative source regarding what is "1 leaf?"
Note: It has come to my attention that one of the common names for the lime leaves common to Southeast Asian cuisine carries a strongly offensive connotation. For more information, I recommend reading this article on The Plate blog from National Geographic, which provides links to several other worthy articles on the subject. I have removed or replaced this terminology from the question and would recommend that users choose to do the same in their comments and answers.
Best Answer
What does "1 leaf" mean?
Since there does not seem to be a clear consensus, this answer will attempt to weigh both positions.
For the sake of clarity, in this answer:
Evidence-based arguments
This section presents sources that address the meaning of "1 leaf" either explicitly or implicitly. The most explicit sources so far tend to fall into the "half leaf" camp.
1 leaf means "full"
fourwindsgrowers.com:
This excerpt is attributed on the above website to a book titled It Rains Fishes: Legends, Traditions and the Joys of Thai Cooking by Kasma Loha-unchit (Pomegranate Artbooks, 1995). See also: thaifoodandtravel.com (below, in the "unclear" section).
1 leaf means "half"
foodsubs.com:
mvcitrus.org.au:
Unclear
thaitable.com:
Sounds like this one is in both camps at the same time. Each leaf is two leaves—very zen.
importfood.com:
This site indicates that there are ~25-30 individual fresh leaves per dry ounce. May need to contact the seller and/or take independent measurements to determine confidently which camp this falls into.
cooksinfo.com:
This could go either way; "[they] look like two" implies "but are actually one," yet the phrase "separate leaf" implies the opposite. Length cited agrees with above, but this could just mean they have a common source (or one is taken from the other).
thaifoodandtravel.com:
The zen approach again. Note: This is written by the author of the book mentioned in the Four Winds Growers entry, above.
theepicentre.com:
specialtyproduce.com:
I was inclined at first to put this in the "half" camp based on "the leaves grow in pairs." However, "they each have a teaspoon shape with pointed ends" gives me pause because the "half" at the stem is typically rounded or cardioid in shape. So the "each" there sounds like it refers to each "full" leaf. The "half" at the stem has no sharp tip, being attached at either end to something else.
boxedupchef.com:
No quote, but interesting to note that this is the first source I've come across that shows an image of the leaves separated into two "halves." Even the sources above that are explicitly in the "half" camp show images of the conjoined "full" leaf.
Practical arguments
This section discusses why (and when) one or the other interpretation might be more appropriate, practically speaking.
1 leaf means "full"
1 leaf means "half"
A better terminology?
I would encourage anyone who shares or writes a recipe calling for lime leaves to clarify exactly how to measure "1 leaf" or to avoid using the term "leaf" at all. Weight measure would be ideal in many cases but can be impractical, depending on the rest of the ingredients in the recipe (and how much time you care to devote to measurement). Some alternatives include:
This recipe uses "stem" instead of "leaf," which is more precise in that two "half leaves" and one "full leaf" both share a single stem.
This recipe refers to both "leaves" and "pairs" to make the quantity explicit.