The vertical alignment, or gradeline, of the road comprises grades and the vertical curves between them. The grade is to the vertical alignment as the tangent is to the horizontal alignment. The gradient is the steepness of the grade. It is usually expressed in percentage form, being the number of metres of rise or fall over a horizontal distance of 100 metres. Although in various texts the terms ‘grade’ and ‘gradient’ are used almost interchangeably, in this book the term ‘grade’ is used throughout as the reference to the straight component of the grade line or profile and the term ‘gradient’ as a reference to its steepness.