Count: 0 — No single-word prepositions begin with the letter Y in standard modern English. This list follows strict criteria: include only single-word prepositions and widely accepted multi-word prepositional phrases whose first word begins with Y, verified against trusted references (Oxford, Cambridge, Merriam‑Webster) and large corpora. Interesting detail: many common Y‑words (you, your, yet, young, yonder) perform other grammatical jobs—pronouns, determiners, adverbs, or adjectives—so they do not qualify as prepositions.

Note the reason: core prepositions in English come from Old English, Germanic, Latin, and Romance sources, and those historical pathways produce few function words that begin with Y. Corpus evidence (COCA, BNC) and major dictionaries show no standard, modern prepositions with initial Y. Consider the near misses: yonder functions as a spatial adverb or determiner in modern use and appears as a regional or archaic form in older dictionaries; yet and yea operate as adverb or conjunction, not prepositions. Tag any Y‑forms you find as archaic or regional rather than include them as standard prepositions.

Consult the broader A–Z list for prepositions under other letters and use corpora or dictionary entries to check frequency and usage. Prioritize single‑word prepositions and accepted multi‑word phrases when building reference lists; treat Y‑initial candidates as non‑prepositional or archaic. Use trusted sources (Oxford, Cambridge, Merriam‑Webster) and corpora to confirm any proposed entries before adding them.