out your app’s user interface by creating a mathematical description of the relationships between the elements define these relationships in terms of constraints either on individual elements, or between sets of elements
attribute is one of left, right, top, bottom, leading, trailing, width, height, centerX, centerY, and baseline. e.g. the left edge of the button should be 20 points from the left edge of its containing view button.left = 1 * container.left + 20 y = m x + c