Here is an oddity: a foreign character appearing in an English book index as the first letter in an indexed phrase. The character is the Greek θ (theta). It appears in that index as the first letter in 5 phrases: θ feature; θ position; θ identification; θ position(s); θ role; θ structure.
Where can you put θ in English?
Where do you put θ in an alphabetical list in English? English uses the digraph <th> to spell sounds represented by θ. Consistently with that fact, this index places the 5 phrases after theme and before tense. That is exactly where these phrases would have appeared if the indexer had spelled them out as theta instead of using the Greek character.
Why use θ instead of theta?
Syntax books (and indexes to syntax books) often write out the name of the letter (theta) instead of the letter itself. So, why did the indexer use the Greek character θ instead of its name (theta)? Maybe that is partly this book—Comparative Syntax of Balkan Languages (edited by María Luisa Rivero and Angela Ralli, 2001)—is about languages used in the Balkan (including Greek). The indexer could reasonably have assumed that using the character itself wouldn’t confuse readers of this book.
Why do these phrases include θ anyway?
The 5 phrases are all technical linguistic terms. They relate to the syntactic roles (thematic roles) that nouns play in a clause. I believe this use of theta (or θ) in some syntactic models started as a concise pointer to the use of theme or thematic in some semantic descriptions.