At the very bottom of the box rendering idea for text presentation we can find a box that is defined inside the current font file in use, namely the character cell box . Each character defined in a font has its own box where the height of the box is equal to the current font-size and the width of the box will depend on the character it self, for proportional fonts, or have a constant and equal width for all characters for a mono spaced font. A series of characters set in succession forms a line box with a height at least equal to the currently defined font-size and such a line box, is in most practical situations, the box with the smallest height that can be used for running text. All elements have a line-height property that, in principle, gives the total height of a line of text.
Space is added above and below the text of the line to arrive at that line-height .
For example, if the text is 12pt high and line-height is set to '14pt', an extra space of 2pt is added, namely 1pt above and 1pt below the line. Empty elements influence these calculations just like elements with content. Half the leading is called the half-leading . After formatting, each line will form a rectangular line-box . The box properties set the size, circumference and position of the boxes that represent elements.