A slow introduction encompasses the same functions as the prologue to an Elizabethan drama, which variously prefigures the outlines of the plot (Romeo and Juliet), acts as an apologia for the limitations of the stage (Henry V) or as a simple elaboration of the title (Doctor Faustus), or sets the scene by outlining what has transpired before the action of the play begins – and, in the case of Troilus and Cressida, by then launching it in medias res:
Now expectation, tickling skittish spirits,
On one and other side, Trojan and Greek,
Sets all on hazard. And hither am I come
A prologue arm’d, but not in confidence
Of author's pen or actor's voice, but suited
In like conditions as our argument,
To tell you, fair beholders, that our play
Leaps o’er the vaunt and firstlings of those broils,
Beginning in the middle; starting thence away
To what may be digested in a play.
Like or find fault; do as your pleasures are:
Now good or bad, ‘tis but the chance of war.
Elizabethan prologues were liminal events, like slow introductions, demanding the attention of a noisy audience, standing between non-event and event, and ushering listeners from the quotidian world to the world of heightened feeling and imagination.
A slow introduction is a special case of a beginning designed to delay a beginning. It was in the tradition of eighteenth-century introductions to promote a sense of wandering. They tend to traverse many keys, sometimes obscure, and involve a thick texture woven out of varied note lengths. Arrival at the “real” beginning generally brings tonal stability, a more transparent texture, and a clearcut theme. The sense of arrival, mingled with relief and renewed anticipation, is palpable. This design was deployed effectively by both Haydn and Mozart, and by the latter part of their careers the slow introduction had become normative.
Eleven of Haydn's last twelve (“London”) symphonies begin with a slow introduction, and so do some of the symphonies before that: three of the six “Paris” Symphonies and three out of the symphonies Nos. 88 to 92.