Additionally, when direct application is not possible, pipes can be tagged with a plastic or metal tag. Tags are also commonly used on pipe valves (not covered in this infographic).
This infographic contains a good overview of the basics of pipe marking. It covers four elements of pipe marking:
- Formatting differences between United States, Europe, and Marine pipe marking, focusing on the US pipe marking standards.
- Color schemes for background colors and the contrasting lettering on the labels (Marine pipe marking also uses colored bands to help further identify the contents of pipes).
- Size requirements for labels and the corresponding text. The labels must be legible and readable.
- Placement of labels. This important aspect of labeling covers how a label should be placed on a pipe run, the specifics of label requirements at fittings and pass-throughs. It also points out the importance of placing the labels where they can be easily seen by the viewer.

Original source of infographic from Creative Safety Supply.