Q:

Do focal plane shutters typically go left-right or up-down?

In modern cameras with focal-plane shutters is there a custom as to which direction they move – whether horizontally and vertically – and on which side they start?
To the extent there is consistency in this feature on the market, what is the reason?
Citing a source from 2000, Wikipedia says:

Most modern 35 mm and digital SLR cameras now use vertical travel
  metal blade shutters. These work in precisely the same way as the
  horizontal shutters, but because of the shorter distance the shutter
  blades must travel (24 mm as opposed to 36 mm), the shutter blades can
  travel across the film plane in less time. This can result in faster
  flash synchronization speeds than are possible with the
  horizontal-curtain focal-plane shutter, and the shutter can reliably
  provide higher speeds....

Makes sense, but wondering if this is still the case.

A:

The overwhelming majority of modern cameras with focal plane shutters go from the top to the bottom in the light box. Since the image projected by the lens onto the sensor is inverted, this means the landscape picture you view later was exposed from the bottom to the top. With portrait orientation all bets are off, since it depends on which way the photographer turns the camera. If the camera has a battery grip with vertical controls that forces you to rotate the camera 90º counter-clockwise (as viewed from behind the viewfinder), the portrait orientation photo when you look at it will be exposed from right to left.

A:

Yes, digital cameras with focal plane shutters generally move vertically across the film plane.
In contrast, most 35mm film cameras with focal plane shutters moved horizontally.  The unexposed film came from a roll at left, and exposed film spooled up at right.  The mechanical system to cock the shutter was simpler and smaller when that cocking motion was in the same direction as the film winding and towards the user-activated lever for that purpose.
The user's mechanical action of advancing the film also cocked the shutter by pulling it towards the winding knob.  During the exposure, the shutter would traverse the film plane right to left by spring action.
In digital cameras, there is no film to move and no mechanical input from the user to advance to the next picture.  The shutter is completely controlled electronically, so is free to be designed in whatever direction makes sense for other reasons.  The most compelling other reason is to have the shutter travel across the image rectangle along its minimum dimension, thereby allowing a shorter overall travel and a shorter X-sync time given otherwise equivalent shutter mechanisms.

