Design context

Paper

Three basic properties of paper- grammage, thickness and bulk. Grammage is usually given in weight per square metre and is a fundamental concept when choosing a paper. Grammage is often used to refer to a papers thickness. This is misleading as thickness actually depends on a papers bulk.
Thickness is measured in micrometres- thousandths of a millimetre- and is the distance between the papers two surfaces. The thickness of the paper will effect the stability and feel of the final printed product.
Bulk, sometimes referred to as volume, defines the relationship between a papers thickness and its grammage. A paper with a low bulk will be more compact and will contain less air than one with a high bulk.  a low-bulk paper is thus thin and heavy, while a high-bulk paper is comparatively light, airy and thick. Because of the lower proportion of air, a low-bulk paper will often be smoother than one with a high bulk, and vice versa.
Roughness is another characteristic of paper to be considered. In general, uncoated papers have a lighter roughness than coated papers. To reduce roughness the paper is compressed and smoothed in a calender or smoothing machine. There is a link between bulk and roughness, since a smooth, compressed paper will have a lower bulk. To achieve a higher bulk, the evenness of the paper surface will have to be compromised to some degree, and the paper will be rougher. Surface roughness may also affect the printing process, where more irregular, rougher papers can be difficult to print.
Opacity or not translucency is another property of paper to consider. The higher a papers opacity, the less translucent it is. A paper of 100 percent opacity is therefore completely non-translucent, while on with a low opacity, such as tracing paper, is more translucent and lets a much higher proportion of light through. During printing, ink penetrates the paper and reduces the opacity, which can result in print being visible through the sheet. This is referred to as ink print opacity. This is an especially important consideration when double-sided printing, particularly on low grammage paper, and you need to be aware of the placement of text and graphics on both sides of the sheets.
The grain direction of a paper describes the orientation of the fibres that make up that sheet. This effects the stiffness of the paper and makes the paper harder to bend along that axis. Grain direction is also revealed by your paper supplier's measurements, as the figure given first refers to the length of the side that runs across the fibre direction.