CC onset clusters are less restricted than CCC clusters. The latter consist predictably of s + voiceless plosive + liquid or semivowel. The former may consist of a combination of any obstruent + liquid or semivowel (with some exceptions), or of s + either a voiceless plosive or a nasal stop.
Source: Joubert 2002: 10
Source: Joubert 2002
Source: Joubert 2002