Poco a poco
Gradually, “little by little”.
What it means
Poco a poco means “little by little, gradually” and designates the course of a change: it is to happen step by step over a longer stretch, not quickly. The phrase is added to another instruction, for instance in poco a poco crescendo or poco a poco accelerando.
It is to be distinguished from poco alone, which weakens the degree of a change. Poco a poco says nothing about the degree but about the distribution: an intensification built up poco a poco can well become large, it merely reaches its goal slowly.
Related is the Latin gradatim, which means a stepwise change. Poco a poco, by contrast, suggests rather a gliding course.
In the notation
The phrase stands in italics before or after the instruction it refers to, usually with a dashed continuation line indicating the stretch. With dynamic changes it stands below the staff, with tempo changes above it. The end is marked by a new marking.
Musical use
The phrase is used with long-laid-out intensifications and reductions: in transitions that lead over many bars to a climax, in codas that gradually thicken, and in closing sections that slowly fade away. In the orchestral music of the nineteenth century it belongs among the most important means of shaping large tension arcs.
It is often extended by strokes or dots after the marking, which show how far the change reaches. It prevents the change from occurring too early or too suddenly. It can extend over many bars, and the climax of the change usually lies at the end.
Performance practice
Divide the stretch before you begin: where is the start, where the goal, how great is the change? Begin very cautiously — the most common fault is driving the change too far already in the first bars. Think in small, even steps. And check, in ensemble, whether everyone has the same division.