"There are 21 consonant letters in the written alphabet (B, C, D, F, G, H, J, K, L, M, N, P, Q, R, S, T, V, W, X, Y, Z), and there are 24 consonant sounds in most English accents. . . . Because of the erratic history of English spelling, there is no neat one-to-one correlation between letters and sounds."
 

Consonant Cluster or Blend

Black / Block / Tree / Frog

"The combination of two consonants together like /st/ is a consonant cluster (CC) used as onset in the word stop, and as coda in the word post. There are many CC onset combinations permitted in English phonotactics, as in black, bread, trick, twin, flat and throw. . . .

"English can actually have larger onset clusters, as in the words stress and splat, consisting of three initial consonants (CCC). The phonotactics of thee larger onset clusters is not too difficult to describe. The first consonant must always be /s/, followed by one of the voiceless stops (/p/, /t/, /k/) and a liquid or glide (/l/, /r/, /w/). You can check if this description is adequate for the combinations in splash, spring, strong, scream and square."
 

"In some instances the consonant cluster may coincide with a cluster which can occur at the end of a word without a suffix; for example the words lapse and laps end with the same consonant cluster and in fact are homophonous, and the same is true of chaste and chased."