Sometimes space is at a premium. Abbreviations, acronyms, and contractions can come in very handy in these instances, but they can also be confusing. Whenever possible, avoid using them. Many times the reason there is not enough space is it isn't being used as efficiently as possible. When you use an abbreviation, please be sure that it is absolutely necessary, that it is both common and clear, and that it is appropriate for your program's target audience. For example, using the octothorpe (# symbol) is an abbreviation for the word 'number' in the English language which fits these criteria in most cases. These same guidelines also apply to acronyms. For example, the acronym CMYK (Cyan, Magenta, Yellow, Black) is acceptable in a color picker for an image processing application designed for graphics professionals, but not in one meant for children. Menus and button labels should never be abbreviated or contain an acronym.
Special care must be used with contractions. They can be a pitfall for users who are not using a program in their native language. Because they require a more advanced command of a language, avoid using them in a place where their role is crucial in conveying the meaning of a message. For example, a checkbox for a message dialog with a checkbox marked "Don't show this message again" which is unchecked by default should be reworded "Always show this message" and have the checkbox checked by default.