The word flute is used to describe many different kinds of woodwind instrument which do not have reeds but rely on the player splitting a thin stream of air against an edge built in to the instrument. The family can be divided into two, the end blown or vertical flutes and the sideways blown or transverse flutes.
The end blown flutes can further be divided into groups that would be those instruments having a fipple or slit and edge device and those which are blown in similar fashion to the transverse flute, let us deal with these first. The pan pipe has a series of tubes, each one to be blown to produce a single note, the pitch of which is set by the length of the tube. These are vertical flutes blown across a round hole, the air stream being split on the edge at the opposite side of the hole. A variation of this instrument is the South American quena which is similarly blown across the end but the opposite side of the hole is shaped to help produce the necessary vibrations. The fipple flutes posses one of the most difficult to make devices found in the flute family, namely the slit and edge. The air is directed to the edge by a channel cut in the roof of the instrument bore, the floor of which is produced by the insertion of a round plug or block into the bore. The edge is at the end of a ramp which slopes down towards the player till it meets the bore. The whistle or flageolet has this mechanism with the floor of the windway being concentric with and at the same diameter as the instrument bore, the block therefore being a simple cylinder, this produces a tightly curved and very strong, stable sheet of air to be split by the edge, the resulting sound being very clear and supporting the higher pitch fractions of the tone. The recorder is manufactured similarly but with the important difference that the windway is positioned outside the bore of the instrument and has a radius equal to approximately twice that of the bore. Recorder blocks have a section which is left standing proud of the main body, this projection fits into the channel in the instrument roof thus bringing the windway floor out from the bore. The resulting tone is softer and less penetrating than that of the whistle.
The transverse flutes have the hole or embouchure cut in the side of the instrument a short distance from the top, the player blows across the instrument splitting the air stream on the opposite side of the hole. There are many systems for making the instrument easier to blow, some have the sides of the hole ramped away to give a flatter edge. The same effect is achieved by cutting the hole as a rectangle with rounded corners. Some employ small raised "bumps" at the sides of the hole to break up the air stream which will miss the edge at either side. There are flutes which increase the radius of the outside of the instrument at the embouchure and yet others employ geometric changes inside to increase the wall thickness. Whatever the technique it is generally agreed that too much time can not be spent studying the shape of this all important hole. The player of the transverse flute can, by changing the way in which the embouchure is formed with the lips, cause the instrument to overblow giving a range of three octaves.