Just bought a type family, and they supplied the font as both ttf and otf. Which do I use?
I am on a mac, using Indesign if that makes a difference?
I thought so. Out of interest, what is the difference?
OTF uses Postscript Type 1 outlines, which use cubic beziers, while TTF uses TrueType outlines, which are quadratic beziers.
The latter is 'simpler' but requires more points to generate some of the same shapes. I think there are also some shapes that can't directly convert from Cubic to Quadratic. Because it requires more points, the filesize can be bigger (though in some cases, it can end up with less points if it's something that is more effectively written as quadratic beziers.)
EDIT: To add, I'm not sure if it's the case for all, but sometimes I found difficulty with certain Mac ttf files and using them in Windows; I think they might have been an Apple true-type font file while the Windows ones I found transport to Mac fine. The .otf files have worked on both without problem, so I prefer those where possible, especially as I don't have to deal with two different files for, like with the old Postscript format.
If you scroll down to the circle approximation on the Wikipedia article, it should show visually the difference:
OTF uses Postscript Type 1 outlines ..
No, OpenType allows either TrueType or Type 1 outlines.
I suspect the font foundry just wanted to make sure to cover all bases -- there is still a good deal of legacy software around that cannot work with OpenType fonts. (I know of one pretty major graphing package that just refuses work with 'em.)
.. difficulty with certain Mac ttf files and using them in Windows; I think they might have been an Apple true-type font file while the Windows ones I found transport to Mac fine ...
There used to be confusion because Apple used their own brand of TrueType, also called "TTF", and indeed 'twas the Tiny differences that made it incompatible with Windows.
Since you have a choice: throw away the TTF and use the OTF.
I thought the OTF extension was to wrap Postscript T1 in, while TTF wrapped Truetype outlines in?
Both of which are Opentype formats though TTF doesn't have to be.
The file extension doesn't determine what type of font it is. You can give a Type 1-flavored OpenType font a TTF extension and it will still work as usual.
(I'm intentionally disregarding the weird Windows issue, where some OTF fonts under some circumstances get an OTF, and others a TTF icon, totally unrelated to its actual contents.)
Ok, though would most vendors stick to the file conventions if they supply both?
Is there any way of telling on Mac OS what outlines an OTF file has? (in windows the preview tells you what outlines are there, but I can't find anything similar on the Mac. InDesign seems to just have an 'o' icon next to both T1 and TT opentype fonts)
Some font management apps tell you which flavour each font is (I know for sure the free Font Explorer X do it).
On a Mac the easiest way to tell is by selecting the font and checking for information (File/Get Info or cmd+i).
If you have an OpenType font with Postscript outlines and the file extension ".otf" and change that extension to ".ttf" the description will change to "TrueType Font" even though nothing else about the font has changed and it still retains Postscript outlines.