The Native American flute is the only flute in the world constructed with two air chambers – there is a wall inside the flute between the top (slow) air chamber and the bottom chamber which has the whistle and finger holes. The top chamber also serves as a secondary resonator, which gives the flute its distinctive sound. There is a hole at the bottom of the "slow" air chamber and a (generally) square hole at the top of the playing chamber. A block (or "bird") with a spacer is tied on top of the flute to form a thin, flat airstream for the whistle hole (or "window"). Some more modern flutes use an undercut either in the block or the flute to eliminate the need for a spacer.