Graphic character

A graphic character, also known as a printing character or a printable character, is a grapheme intended to be rendered in a form that can be read by a human. In other words, it is any encoded character that is associated with one or more glyphs. (It is thus distinct from a control character, one that is acted upon and not displayed.)

ISO/IEC 646

In ASCII (specified in ISO/IEC 646), graphic characters are contained in rows 2 through 7 of the code table. However, two of the characters in these rows, namely the space character SP at row 2 column 0 and the delete character DEL (also called the rubout character) at row 7 column 15, require special mention.

The space is considered to be both a graphic character and a control character in ISO 646.^[1] It can be considered as a character with a visible form or, in contexts such as teleprinters, a control character that advances the print head without printing a character.

The delete character is strictly a control character, not a graphic character. This is true not only in ISO 646, but also in all related^{[clarification needed]} standards including Unicode. However, many other character sets deviate from ISO 646, and as a result a graphic character might^[a] occupy the position originally reserved for the delete character.^[b]

Unicode

In Unicode, Graphic characters are those with General Category Letter, Mark, Number, Punctuation, Symbol or Zs=space. Other code points (General categories Control, Zl=line separator, Zp=paragraph separator) are Format, Control, Private Use, Surrogate, Noncharacter or Reserved (unassigned).^[2]

Spacing and non-spacing characters

Most graphic characters are spacing characters, which means that each instance of a spacing character has to occupy some area in a graphic representation. For a teletype or a typewriter this implies moving of the carriage after typing of a character. In the context of monospace typefaces (or computer fonts), each spacing character occupies one rectangular character box of equal sizes.^[c]. If a text is rendered using proportional fonts, widths of character boxes are not equal, but are positive.

There exist also non-spacing graphic characters. Most of the non-spacing characters are modifiers (called combining characters in Unicode), such as diacritical marks. Although non-spacing graphic characters are uncommon in traditional code pages, there are many such in Unicode. A combining character has its distinct mark, but it applies to a character box of another character, a spacing one. In some historical systems such as typewriters, this was implemented as 'backspace and overstrike'.

Note that not all modifiers are non-spacing – the Unicode block "Spacing Modifier Letters" list a number of others.

Notes

1. as is the case in code page 437 and related standards
2. This does not mean the delete character is absent; it just means 0x7F is overloaded, and outputting it will either print the graphical character or perform a deletion, depending on the routine used. For example in most BASIC implementations, using the PRINT command with 0x7F will delete, but using POKE will output the graphical character.
3. Or maybe two adjacent ones, for non-alphabetic characters of East Asian languages