If one character is using sign language and lip reading while the other is speaking normally, how do you represent the lines of the former? Quotations with "he signed" attribution or italics? E.g.
"When did you first feel the pain?" asked the doctor.
Two days ago, signed the patient.
Italics make sense when sign is not the language most of the dialogue is in (I think this is a common tactic for "secondary" languages). But if sign is the main language in your piece, I would just put it in quotes like any other dialogue.
I guess the only question I would have for you is what is the main language of the dialogue? If it's sign language, I wouldn't give it second class treatment in italics.
Yes, italics is exactly how I'd do it. Dean Koontz did that in Watchers, as I recall.
You could also use normal text set off by "guillemets", which are symbols used in some other languages instead of quotation marks. See the Wikipedia article 'Guillemet'. There are several styles, e.g. with or without extra spaces, with the guillemets pointing inwards » « instead of outwards, « » etc.
It would look like:
"Hi Dave."
« Good morning Sarah. »
This is distinctive, and wouldn't use italics for another meaning in addition to emphasis, thought that are not spoken out loud, etc.
When it comes to actually writing the signer's line, I tend to write them between apostrophes. I find it easier and a little more visually appealing than italics, but still allows the reader to differentiate the two languages. I more commonly save the italicization for internal thoughts.
'I wish I could go to the park.' vs. I wish I could go to the park.
I find italics to be easier to understand as the reader. It makes it less complicated and can be caught by the eye pretty rapidly and the reader will learn to recognize the person's dialogue by the italics. It's just overall a lot simpler.
Sign language is not merely a representation of English with signs but a language of its own. For a brief introduction refer to the section on the relationship between sign and spoken languages in the Wikipedia article on sign languages.
Therefore, if you represent sign language in an English text, the English words are in fact a translation from a different language. Treat it as such. Specifically:
If all (or most) characters in your text communicate in sign language, use quotes.
Think of a book written in English but set in France with the characters speaking French. For the sake of your English readers you will render all dialogue in English, as if the characters were speaking English. Do the same when the story is set in a community of signing persons who use sign language to communicate.
If most of your chacters communicate in English most of the time and sign language is the exception, markup the English representation of sign language as italics.
Non-spoken language – such as written text or speech from a device (e.g. radio) – are conventionally marked up as italics. Sign language is not spoken, so use the markup for non-spoken languages.
I think it would make the most sense to use the primary language in quotations, because as the previous answer stated, using too many italics would end up making the literature seem cluttered.
