written in flat keys. The keys of Concert F, B♭, E♭, and A♭ are the most frequently used. (NOTE: These refer to the key the march in, not the modulated key in the trio (see below)). March music forms Most marches follow a fairly strict structure known as the march music form, the origins of which appear to be derived from the sonata form, as these two forms share similar ideas of contrasting sections. The period of early development of the true march music form denotes the start of the march music era in the US by the 1850s; eventually the form was standardized by John Philip Sousa. While the form varies among different styles of march music, all marches have these common elements: Different sections, called strains. Several separate melodies. A middle section, dubbed the Trio, that features contrasting melodic material and is usually lighter in texture and more lyrical in style. After the Trio the main section is recapitulated. Following are descriptions of various march forms that have been popular and are/were frequently used by march music composers. Military march form The military march form is largely credited to John Philip Sousa, who came to be known as "The March King". He standardized the "military" form (as compared to the "regimental" form), using it in over half of his marches. The military march form is: I-AA-BB-C(C)-Br-C-Br-C(Grandioso); or, in more generic code: I-AA-BB-CCDCDC. The first section of a military march is called the introduction (I) or fanfare; it is typically 4, 8, or 16 bars long and played in marcato style, using forte (loud) dynamics and chromatic alterations to catch the attention of the listener. The introduction is usually the shortest section of a march and is almost never omitted. Still, examples of marches written without an intro include "Bugles and Drums" and "The Footlifter". Introductions to marches vary but some standard practices include: (a) tutti unison ("The Washington Post"), (b) tutti rhythmic unison with contrary motion ("The Thunderer"), (c) tutti rhythm unison in four-part harmony ("Semper Fi"), and (d) four-part independent ("The Klaxon"). The introduction is commonly based on the dominant key to create clarity of key as centered in the first strain, (see harmonic progressions below). Generally, the intro is not repeated, but it is in some marches: "Bravura", "The Rifle Regiment", and "Washington Grays". The introduction generally starts in major because marches typically are in major