Main spellings (~95%)
⟨sh⟩ ~40%
The most recognizable spelling — used in native English words.
⟨ti⟩ ~30%
In words from Latin, ⟨ti⟩ before a vowel makes /ʃ/. Very common in -tion endings.
The -tion ending: The extremely common -tion suffix is always pronounced /ʃən/. Think: nation, station, action, question, information, education. This is one of the most reliable spelling patterns in English.
⟨ci⟩ ~10%
In -cial, -cious, and similar endings.
⟨ssi⟩ ~10%
In -ssion endings.
⟨si⟩ ~5%
In -sion endings (when not -ssion).
-sion vs -ssion: Both -sion and -ssion make the same /ʃən/ sound. The spelling depends on the base word: tense → tension, miss → mission, permit → permission.
Unusual spellings (~5%)
⟨ch⟩
In words from French, ⟨ch⟩ makes /ʃ/ instead of /ʧ/.
French ⟨ch⟩: Words borrowed from French often keep the French pronunciation where ⟨ch⟩ = /ʃ/. Examples: machine, chef, champagne, charade, chic, niche. If a word looks French, try /ʃ/.
⟨s⟩
Before ⟨u⟩ in some words, plain ⟨s⟩ makes /ʃ/.
S before U: In "sure" and "sugar" (and their derivatives), ⟨s⟩ is pronounced /ʃ/. This is an unusual pattern — most ⟨su⟩ combinations are /s/ + vowel (sun, super, such).
⟨sci⟩
In a few words, ⟨sci⟩ makes /ʃ/.
⟨sch⟩
In words from German, ⟨sch⟩ makes /ʃ/.
German ⟨sch⟩: German words spell /ʃ/ with ⟨sch⟩ (Schmidt, schnapps, kitsch). But be careful — in Greek-origin words like "school" and "scheme," ⟨sch⟩ is /sk/!