Originally I had placed this chapter with the tafkhim (thickening) and tarqiq (softening) of the alif, then found it better off bundled with the other letter sets. In sum, there are four letter sets:

Hams - فحثه شخص سكت

Qalqala - قطب جد

Isti'laa' - خص ضغط قظ

Tasfeer - ز س ص