alef - mother over six of the aspirated variants of the double letters

mem - mother over six of the muted variants of the double letters and other stops (alef, eyin, tet, qof)

shin - mother over the sibilant letters (zeyin, samek, tsadi, and shin)

There are eight letters remaining:

Two glides/semivowels: yod, vav

Two liquids: resh, lamed

Two nasals: mem, nun

Two gutturals: heh, khet

Of all the letters, resh is particularly elusive, as it has traits of a guttural, a double letter, and even somewhat resembling a sibilant (when trilled), a mute/stop (when tapped), or an aspirant when rhotic.