# `GSUB`: Glyph Substitution table

The `GSUB` table is an OpenType table.

class fontTools.ttLib.tables.G_S_U_B_.table_G_S_U_B_(tag=None)

  Bases: `BaseTTXConverter`

  Glyph Substitution table

  The `GSUB` table contains glyph-substitution rules used in OpenType Layout.

  See also https://learn.microsoft.com/en-us/typography/opentype/spec/gsub

  closure_glyphs(s)
  merge(m, tables)
  mergeMap = {'table': <function mergeObjects>, 'tableTag': <function onlyExisting.<locals>.wrapper>}
  neuter_lookups(lookup_indices)

    Sets lookups not in lookup_indices to None.

  prune_features()

    Remove unreferenced and duplicate features in FeatureList Remove unreferenced features and remap duplicate feature indices in ScriptList and FeatureVariations

  prune_lookups(remap=True)

    Remove (default) or neuter unreferenced lookups

  prune_post_subset(font, options)
  prune_pre_subset(font, options)
  remap_duplicate_features(feature_indices)

    Return retained feature indices(without duplicates) and remapped feature indices

  remove_redundant_langsys()
  retain_empty_scripts()
  subset_feature_tags(feature_tags)
  subset_glyphs(s)
  subset_lookups(lookup_indices)

    Retains specified lookups, then removes empty features, language systems, and scripts.

  subset_script_tags(tags)