This is the first period where we can begin to be fairly certain as to how a great deal of the music which has survived actually sounded. The earliest written secular music dates from the 12th century troubadours (in the form of virelais, estampies, ballades, etc.), but most notated manuscripts emanate from places of learning usually connected with the church, and therefore inevitably have a religious basis.
Gregorian chant and plainsong which are monodic (i.e. written as one musical line) gradually developed during the 11th to 13th centuries into organum (i.e. two or three lines moving simultaneously but independently, therefore almost inadvertently representing the beginnings of harmony). Organum was, however, initially rather stifled by rigid rules governing melody and rhythm, which led ultimately to the so-called Ars Nova period of the 14th century, principally represented by the composers de Vitry, Machaut, and Landini.
The fifteenth century witnessed vastly increased freedoms, most particularly in terms of what is actually perceived as 'harmony' and 'polyphony' (the simultaneous movement of two or three interrelated parts). Composers (although they were barely perceived as such) were still almost entirely devoted to choral writing, and the few instrumental compositions which have survived often create the impression (in many cases entirely accurately) of being vocal works in disguise, but minus the words.
There is obvious new delight in textural variety and contrast, so that, for example, a particular section of text might be enhanced by a vocal part dropping out momentarily, only to return again at a special moment of emphasis. The four most influential composers of the fifteenth century were Dunstable, Ockeghem, Despres and Dufay.
The second half of the 16th century witnessed the beginnings of the tradition which many music lovers readily associate with the normal feel of 'classical' music. Gradually, composers moved away from the modal system of harmony which had predominated for over 300 years (and still sounds somewhat archaic to some modern ears), towards the organisation of their work into major and minor scales, thereby imparting the strong sensation of each piece having a definite tonal centre or 'key'.