Some characters in Arabic script are signs that span (or enclose) numbers, such as U+06DD End of Ayah and U+0605 Number Mark Above:

Spanning sign examples

Over time these have been variously called:

• prepended concatenation marks
• subtending marks
• prefixed format control characters

For the purposes of this document, we will use the term spanning signs (or more simply, signs) to refer to these characters and sign glyphs for the glyphs within the font that will represent them.

Since digit choice is dependent on regional use, these marks may be used with European digits (U+0030..U+0039), ARABIC-INDIC digits (U+0660..U+0669) or with EXTENDED ARABIC-INDIC digits (U+06F0..U+06F9).

From a practical standpoint there isn’t a need to support an arbitrary-length sequence of digits. For SIL fonts, we have found the maximum number of digits as documented in the following table to be sufficient for most uses.

Examples of how these appear are shown below using varying numbers of digits with hamza used as a separator.

For more Unicode information about these characters, see Signs Spanning Numbers in Section 9.2 and Prepended Concatenation Marks in Section 23.2 of The Unicode Standard.