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In typography, width of a block of typeset text

In typography, line length is the width of a block of typeset text, usually measured in units of length like inches or points or in characters per line (in which case it is a measure). A block of text or paragraph has a maximum line length that fits a determined design. If the lines are too short then the text becomes disjointed; if they are too long, the content loses rhythm as the reader searches for the start of each line. Line length is determined by typographic parameters based on a formal grid and template with several goals in mind: balance and function for fit and readability with a sensitivity to aesthetic style in typography. Typographers adjust line length to aid legibility or copy fit. Text can be flush left and ragged right, flush right and ragged left, or justified where all lines are of equal length. In a ragged right setting, line lengths vary to create a ragged right edge. Sometimes this can be visually satisfying. For justified and ragged right settings typographers can adjust line length to avoid unwanted hyphens, rivers of white space, and orphaned words/characters at the end of lines (e. g., "The", "I", "He", "We").

Traditional line length research, limited to print based text, gave a variety of results, but generally for printed text it is widely accepted that line lengths fall between 45 and 75 characters per line (cpl), though the ideal is 66 cpl (including letters and spaces). For conventional books line lengths tend to be 30 times the size of the type, but between 20 and 40 times is considered acceptable (i. e., 30 × 10pt font = 300 pt line). For printed works with multiple columns, 40–50 cpl is often better. Punctuation should preferably hang outside the measure. Generally, if the measure is wide, the leading of a text should be increased—if the measure is short, it can safely be decreased. Reverse text, i. e. white text on black, also requires more leading. The experience of the reader can also be considered as a factor when determining the count of characters within text lines. For novice readers, text lines should contain between 34 and 60 characters, 45 being the optimal number. Texts for expert readers could contain between 45 and 80 characters, with an optimal count of 60 characters.

Screen reading poses additional challenges, making the adoption of traditional line length research to the digital format problematic. Unlike printed text, writing for digital media must accommodate factors such as glare, flicker, and scrolling/paging. The measure is the number of characters per line in a column of text. Using CSS to set the width of a box to 66ch fixes the measure to about 66 characters per line regardless of the text size as the ch unit is defined as the width of the glyph 0 (zero, the Unicode character U+0030) in the element's font. For user agents that do not support the ch unit a value of 33em can be used as, on average, one character takes up 0.5 of a typographic em. On the other hand, there have been studies indicating that digital text at 100 cpl can be read faster than text with lines of 25 characters, while retaining the same level of comprehension.

There are a few methods to calculate line length to fit the intended average count of characters that such lines should contain based on the factors listed above. Most, if not all, of these methods use the length of the lowercase alphabet (LCA) as a reference for its calculation. The lowercase alphabet (a measurement of the array of characters of the hegemonic roman alphabet from a to z in typographic points) was sometimes included in type specimen booklets. If not available, the first step to calculate the line length for all these methods is the measurement of the LCA at the size that will be ultimately used. The first of these methods consists of an adjacency matrix that positions the LCA in points on the x axis and the line length in picas on the y axis. The matrix is used by locating the number closest to the previously calculated LCA on the left column of the matrix and then scanning across the columns the number of characters that one would like to set in the text line. Once the number is located, the top row of the selected column will indicate the ideal line length.