First set of sounds children are taught in reception as part of Read Write Inc.
set2 RWI sounds
Once children are familiar with set 1 sounds and can segment and blend (split up and push sounds together) to read short words eg. run/ pat children are introduced to set 2 sounds. They are taught the phrase "Two letters one sound" or "Three letters one sound" as each sound is written as (diagraph) has 2 or 3 letters.
set3 RWI sounds
When ready, children move on to set three sounds. They are taught alternative ways to write familiar sounds and that the same sound can be written in a number of ways for example: ow / oa / o_e. They are also introduced to split digraphs where the sound is split into two parts for example a_e - cake, bake, shake. They also learn the written letter can produce a different sound for example ow for 'blow the snow' and ow for 'brown cow' same grapheme (written sound) but different pronunciation.