The single piece, hardwood whistle with steel mouthpiece, is tuneable. Usually includes wooden case. The PCV plastic whistle is a loud, tuneable 2 piece cylindrical whistles. Good tone and intonation, moderate chiff. The pitch of a regular tin whistle can be altered by sliding the barrel in and out of the head.
Many whistles have the head glued or securely jammed into the barrel.
Be careful not to overheat, or the plastic head will melt. If you get the head off, smear it with vaseline or grease to make further tuning easier. Another possibility is to hold a lighter under the joint while turning it, for 5-10 seconds. The plastic will blacken, but this can be cleaned off later. Both of these techniques have been used successfully with the glued-together Generation whistles.