It's neither a dash nor an underscore (_), the hyphen (-) is a punctuation mark most commonly used to join two words to make one - and for such a small character, it has an important role in any text document. In fact the hyphen has an entire function dedicated to its use in Word 2007.
Hyphenation deals with the role of the hyphen when used to separate words at the end of a line. A word that appears at the end of a line, but is too long to fit completely on that line, can be hyphenated. This is especially helpful if turning the word to the next line avoids unsightly gaps in the text on the previous line.
Obviously the difficulty is that, when inserting a hyphen in a word at the end of a line, it does not form a permanent part of the spelling of the word. The rules for deciding where to insert a hyphen, therefore, can sometimes cause confusion. Some prefer to divide words between consonants - for example 'splen-dour'; and some between vowels - 'appreci-ate'. Words of one syllable should never be divided - for example 'rhythm'. As a rough guide, the hyphen should be positioned in the word at a place that eases reading the text.
Word 2007 has a sophisticated hyphenation function, which undertakes most of the decision-making for you. The hyphenation feature can be used to prevent gaps in lines when text is justified, or to make text more equal on lines when using 'ragged' copy. Hyphenation also prompts a better understanding and awareness of the shape of the words on a line, and enables the author to create a more professional and polished looking document.
The hyphenation feature can be set to automatically or manually hyphenate text, insert optional or non-breaking hyphens, and set the maximum amount of space allowed between a word and the right margin without hyphenating the word.
By selecting Automatic Hyphenation, when a word is too long to fit on to the end of a line, Word 2007 automatically hyphenates the word over two lines. Automatic Hyphenation is a great facility if you are confident of where a hyphen will be inserted. If the text is edited, then Word automatically re-hyphenates the altered text as required.
Manual hyphenation gives the author more control to select where a hyphen should or should not appear. Again if you edit the text, Word will offer any alternatives to new line breaks, and will not automatically hyphenate words. This is particularly useful when using compound nouns and verbs such as 'in-house' or 'e-mail' and where house style dictates the spelling.
An optional hyphen can be used when you want to ensure that a word will only be hyphenated at the end of a line in a certain place. For example, to ensure that the word nonhyphenated is never broken as 'nonhyphen-ated'. An optional hyphen can be inserted after 'non' and before 'hyphen'. To insert an optional hyphen, use the keys 'CTRL+HYPHEN'. To view optional hyphens, select the function Show and Hide from the Paragraph group on the Home tab.
A non-breaking hyphen can also be inserted in words or phrases that you do not want to be broken at the end of a line. For example in the sequence of numbers, 'Telephone: 555-5555' the telephone number should not be broken over a line.
A non-breaking hyphen can be typed to ensure that the number will not be split over two lines. To insert a non-breaking hyphen, click where the hyphen appears and key 'CTRL+SHIFT+HYPHEN'. Now the entire number will move on to the next line and will not be split at the hyphen point.