Question: I have some photos in a standard 2:3 aspect ratio that I want to print and put in a frame. However, all the options for printing as well as the standard frame sizes I found are not *exactly* in a 2:3 ratio, but slightly off. 

For example, here are some standard sizes I found that are suggested by both the photo printing service I use and the frame sizes that are available at some online stores: 

* 9cm x 13cm
* 11cm x 17cm
* 13cm x 18cm

But those aren't actually 2:3 ratios. The exact length for the long side should be `9 x 1.5 = 13,5`, `11 x 1.5 = 16,5` and `13 x 1.5 = 19.5`. So why are the ratios off like that? I first thought it's because of the bleeding or something like that, but that doesn't make sense either since the actual ratio for those formats vary and the cutoff differs between them (as well as between the long and the short edge).

So what's up with those standard sizes? And how will they fit in the frame? I had some pictures printed out at 11x17 cm, I've measured them now and they are actually more like 11.2x17 cm, which is pretty much 2:3 ratio. What's up with that?

For example, for this purpose I would like to get the 13cm frame & print. However, between the mentioned 13x18 format and an actual 2:3 format is some disparity. So what will happen? Will the print service cut the long edge by 1.5 cm? Will the short edge be slightly shorter? Or what?

Answer: 
> 
> I have some photos in a standard 2:3 aspect ratio that I want to print
>  and put in a frame. However, all the options for printing as well as
>  the standard frame sizes I found are not exactly in a 2:3 ratio, but
>  slightly off.
> 
> 
> 

If you want to print and frame at your own chosen aspect ratio, you can do two things:

1. Print smaller picture on a larger paper. For example, print 10x15 on a 13x18 paper. The white border around your image is necessary in some frames where the photograph is mounted to the front mate anyways.
2. Use custom sized framing mats or even custom sized frames. A framing shop should be able to help with both.
Lots of *stuff* is "not exactly ..." because of historical compromises. Many of these involved adapting something new to something old.