"Text on the computer screen is hard to read not only because of the low resolution of computer screens but also because the layout of most Web pages violates a fundamental rule of book and magazine typography: the lines of text on most Web pages are far too long for easy reading." -- The Web Style Guide.

Our eye's span of acute focus is only about 3 inches (8 cm), typically enough to see three to five words wide and high when printed text is held at approximately 18 inches (46 cm) from the eye. As the eyes follow a long line of text, they tend to lose track of the next line. Thus magazines and newspapers use columns to help maintain this comfortable focus.

Left to its own, the width of your text will vary depending upon the user's browser window and the font size the user chose.

If you want to control how wide your column of text is, use an invisible table (border="0") and specify the table size in pixels (width="300"). Or you can use CSS to control the paragraph width ( p { width:300px;} ).

An average of 10 to 15 words per line is the best balance between space efficiency and legibility. Increasing the line spacing proportionally to the width of the passage also helps the eyes correctly identify the next line.

You can use CSS to control the line spacing ( p {font-size:12pt; line-height:16pt;} ).

Barring all the technical tweaks, paragraphs can be broken up into more readable segments using paragraph spacing, lists and visual elements.