I am new to this and I am looking to obtain a set of pipes. I have been reading a lot about Flat Pipes. Could someone explain the difference between these and a concert D set in a relatively simple manner. Is one set better to play in sessions with than the other? Is one set easy to play than the other for a beginner?
Teflon
D is the key you would most likely take to a session to play with others. There is a poll thread where it appears most people are getting D sets. http://chiffboard.mati.ca/viewtopic.php?t=21504&sid=e306786044e7183f7c6c030d0467f271
There’s been lots of changes in tuning over the last couple of centuries. What is B today may once have been C or C#, but today’s tuning for D is what is called “concert pitch”. Anything else (what may have once been considered to be in D) is now called a flat set, so the term really refers to anything in Bb to C#, i.e. flat of D.
Fingering is closer to whistle, except the the back D’ hole played with the thumb. Unlike highlands, UP has two full octaves, and really good chanters will go up into the third octave, as well. When fitted with extra keys, the UP chanter is fully achromatic.
You can feel the pull, can’t you? That irresistable calling to take the plaid skirt off and come on over to … the Dark Side. Bwa-ha-ha-ha-ha! 
djm
Aww, don’t sentence him to a wide-bore set just yet! 
Here’s an article from the Seattle-based Irish Pipers’ Club publication, “The Pipers’ Review/Iris na bPiobairi”, on flat sets:
http://www.irishpipersclub.org//Web_art2.htm
- Bill
Teflon,
There are 2 major differences between a standard concert-pitch wide bore set, and a narrow-bore flat set.
A concert pitch D set will be the one that you will go with what 95% of sessions are doing. These are the pipes you will play with other people.
There are flat sessions, but these are a rarity, not the norm.
Concert pitch pipes generally are wide bore pipes which makes them significantly louder than flat pitch pipes.
Flat pitch pipes would generally be in C#, C, B or Bb. Also, there are some narrow-bore D sets which play at concert pitch, but are narrow bore so they are quieter. The lower pitched sets also require a bit more of a finger stretch since the chanters are longer.
The two sets of pipes have different sounds to them – to put it simply, flat pitch pipes are quieter and more mellow.
If you want to play with other people, and if you want to ever be able to take lessons at a tionol or summer school, get a D set or at least a D chanter.
That being said – flat pitch pipes are awesome sounding and are usually less of a bear to play than concert pitch pipes.
This is the short answer. There are a many subtle differences, and there are historical reasons why both types of pipes exist, but you asked for the simple version 
Hope that helps,
Pelham
Here’s my simple answer (since that was what was requested):
Get a flat set - you won’t be sorry.
I have a concert set, but I’m wishing it were a Bb or a C.
Figure the time you’d actually spend playing in sessions, vs. the time you’d spend playing on your own… you’ll probably conclude that session time is a fraction of the time playing alone.
Further - if you have a nice flat set, you can always buy a “D” chanter to use with it and play in sessions (no drones, of course) and that would fill your session needs.
Finally, it’s incorrect to imply that a flat set could never play with, say, a fiddler - listen to the CD “Kitty Lie Over” with Mick O’Brien, Caoimhín O Raghallagh and see how good a flat piper sounds with a fiddler who’s tuned to the pipes. It’s incredible.
There’s my $0.02.
Achromatic?
Acrobatic?
Aromatic?
I think of the “family of keys” that upipes come in kind of like the saxophone family: clearly they are all related, but each member has its own character.
t
…
Yeah, my family’s like that, too.
achromatic: being without accidentals or modulation
Is that not the right word? Not sure what the is about.
djm
djm, you wrote that a keyed chanter would be achromatic, where “chromatic” would be its actual condition (if fully keyed, of course). I found “achromatic” to be deliciously apt, the discussion being about pipes, and all the quirks implicit to them.
Come to think of it, “preterchromatic” might serve best!