Suppose we have four straight lines, each equal in length to the others, each in contact with one of the others at each of its endpoints, and each at right angles to the lines with which it is in contact.
We can see that these lines form a square. And if we consider the square as the boundary formed of the four lines (it would also be possible, and perhaps more common, to consider the bounded area as a square), then the lines are parts and material causes of the square. And those parts cause the whole by having squareness, the formal cause.
What is squareness? Someone might say it is simply the properties described in the first paragraph above. And this is close to the truth, but it is not exactly right. For a square is one figure, and the properties described are many, and insofar as they are many, they do not sufficiently explain the unity of the square. Squareness is rather the shape that a thing has which has those properties; and that shape is one shape, not many shapes.
The properties described, then, are not squareness itself, but can be called the disposition to squareness. By having these properties, the four lines are disposed to have squareness, and consequently to form a square.