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In text shaping, a cluster is a sequence of characters that needs to be treated as a single, indivisible unit. A single letter or symbol can be a cluster of its own. Other clusters correspond to longer subsequences of the input code points — such as a ligature or conjunct form — and require the shaper to ensure that the cluster is not broken during the shaping process.
A cluster is distinct from a grapheme, which is the smallest unit of meaning in a writing system or script.
The definitions of the two terms are similar. However, clusters are only relevant for script shaping and glyph layout. In contrast, graphemes are a property of the underlying script, and are of interest when client programs implement orthographic or linguistic functionality.
For example, two individual letters are often two separate graphemes. When two letters form a ligature, how