Gap can label segments of readings and contigs using "tags". The program recognises a set of standard tags types and users can also invent their own. Each tag type has a unique four character identifier, a name, a direction, a colour and a text string for recording notes. Tags can be created, edited and removed by users and by internal routines. Tags can also be input along with readings. For example all readings can be screened for Alu segments prior to assembly using repe (see section repe) which adds an appropriate record to the readings experiment file. This information is copied into the database and becomes a tag attached to the reading.