I know nothing about how to play them but I’ve been watching some videos of folks playing the Uilleann pipes and I notice they often purposely quickly lift and lower the chanter as they’re playing. I’m just wondering why.
Because they can’t believe they’ve chosen to play such a ridiculous instrument, and they have to keep poking themselves in the knee to remind themselves what idiots they’ve been.
Either that, or closed fingering / open fingering.
But I think the first explanation is more likely.
This is the #1 question I get. For starters, that’s how you get the bottom note on the chanter. There’s more to say on this…
Yes people ask about it all the time… it’s the second thing they notice, after the bellows!
The reasons for lifting the chanter off the leg boil down to
-
to sound Bottom D
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to change the volume, pitch, and timbre of various higher notes
You could think of the opening at the chanter’s bottom as an additional fingerhole, and the leg as an extra finger.
Sometimes the effect of lifting the chanter is not what you might expect. One thing I often demonstrate, to people interested in the acoustics of the thing, is how F# in the upper octave lowers to F natural when the chanter is lifted, but E rises to F natural when the chanter is lifted. (At least on my chanter.)
Interesting responses from you surly pipers.
To me, as an observer, the fundamental reason for end stopping the chanter is to make closed fingering possible. After all, it’s the ability to play staccato articulations which sets the uilleann pipes apart from most other pipes, and which is at the heart of the uilleann piping sound.
The other things like hard D and timbre are obviously important, and may be what you actually think about most when driving the chanter. But I wonder if seasoned pipers tend to take open/closed fingering for granted and overlook it as more basic.
To the casual listener, I think that the unique sound of closed piping is more likely to catch the ear than the subtleties of hard D and the rest. But I could be wrong. 
Regardless of more open or closed (which has more to do with how many fingers are on the chanter and the silences by closing the chanter with all fingers on and closed on the knee) playing the chanter must be lifted to sound the fundamental
D note. Or as pan celticpiper points out to add color to a note. Swell if you will.
Generally more closed playing above D is on the knee, but not necessarily.
Ah I just muddled it again but open or closed you need to lif ththe chanter for d.
Your post is a good response, though to a different question, what is the reason for stopping the chanter on the leg.
I addressed the question which was asked, was what is the reason for lifting the chanter.
How’s that for surly 
In my opinion, the casual listener is more likely to be caught up by fast, flamboyant playing rather than the tonal qualities of the pipes or the technical mastery of the piper.
Incidentally, I occasionally play in public with a harper. I frequently get asked why I lift the chanter off the knee and the response is (1) to get bottom D and (2) to colour other notes. The harper frequently gets asked why the strings of his harp are different colours!
what is it about back d that means it has to be played on the knee?
Oooh, I like that, Richard. You win the surly smartass award of the day. And that’s no mean feat. 
But that makes sense, and you’re right … Now I see that that is the exact question that people are answering. Whereas I suspect JTC’s question is more general, as to why the chanter is on and off the knee at all.
The back d can be played off the knee of you lift the C finger also. It depends on the chanter of course.
Patrick
I suspect that this is WAY more information than Jim thought he would get, but it is also a fundamental reason why I love the pipes: so much depends on such a seemingly small gesture.
Ain’t that the truth!
So let me tell you what just happened… I was looking at the listings in the used instrument section and saw the 3/4 set for sale, clicked the link and watched the video demonstration that’s there. As I’m watching, I’m wondering why pipers keep lifting the chanter off their leg and I thought to myself, “I should ask the folks in the C&F Uilleann Pipe section” …took a few sections for the deja vu to kick in, but I eventually remembered I already had but forgot that I had.
Anyway, I now understand why it’s done. Thanks for all the responses, both surly and the other kind …very surly.
I’ve just caught up with this, because of your late post JTC111. And I’m glad I did - very clear responses from the pipers there. 
… and yet still I have a further - possibly ignorant - question for you guys. The description of lifting the chanter off the knee being like an extra finger hole makes total sense to me. So I’m just wondering if it’s analogous, in general playing as opposed to having to for the bottom D, with the Eb key on a flute, with most leaving it closed, but a few (like me) venting it (equivalent to raising a chanter off the knee?) on every note except D. Am I close? Or way off?
Good question. At first, I’d have said, no, it’s not really analogous. But there are some similarities… Makes me think.
At any rate, lifting off the leg does change tone and tuning of notes above, so in that respect it is similar. However, it also provides a larger variety of options, more than the Eb key anyway. But that’s probably just because it is a chanter.
**sur·ly (sûrl)
adj. sur·li·er, sur·li·est
- Sullenly ill-humored; gruff.
- Threatening, as of weather conditions; ominous: surly clouds filled the sky.
- Obsolete Arrogant; domineering.**
Just had to do a quick check..
yeah, it’s still a pretty fitting description!
Surly to bed,
Surly to rise,
Makes a man obsessive, impoverished,
One of those uilleann piper guys.