Question: What is attached to the bag using a coupler?
About the article "Cake", section "Cake decorating".

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The Riblet short-slot coupler is two waveguides side-by-side with the side-wall in common instead of the long side as in the Bethe-hole coupler.
A slot is cut in the sidewall to allow coupling.
This design is frequently used to produce a coupler. 
A star coupler is a device that takes in an input signal and splits it into several output signals.
In fiber optics, and especially in telecommunications, a star coupler is a passive optical device, used in network applications.
An optical signal introduced into any input port is distributed to all output ports.
Because of the way a passive star coupler is constructed, the number of ports is usually a power of 2; "i.e.
", two input ports and two output ports (a "two-port" coupler, customarily called a "directional coupler," or "splitter" ); four input ports and four output ports (a "four-port" coupler); eight input ports and eight output ports (an "eight-port" coupler), "etc". 
The C-AKv coupler is a newer compact Willison coupler developed by Faiveley Transport.
It is mechanical fully compatible to the SA-3 coupler and the Unicoupler and if additional buffers are mounted it can be coupled with the conventional European screw coupling as well. 
A device called a coupler allows the pipes of one division to be played by a manual.
For example, a coupler labelled "Swell to Great" allows the stops of the Swell division to be played by the Great manual.
It is unnecessary to couple the pipes of a division to the manual of the same name (for example, coupling the Great division to the Great manual), because those stops play by default on that manual (though this is done with super- and sub-couplers, see below).
By using the couplers, the entire resources of an organ can be played simultaneously from one manual.
On a mechanical-action organ, a coupler may connect one division's manual directly to the other, actually moving the keys of the first manual when the second is played.
Some organs feature a device to add the octave above or below what is being played by the fingers.
The "super-octave" adds the octave above, the "sub-octave" the octave below.
These may be attached to one division only, for example "Swell octave" (the "super" is often assumed), or they may act as a coupler, for example "Swell octave to Great" which gives the effect while playing on the Great division of adding the Swell division an octave above what is being played.
These can be used in conjunction with the standard eight foot coupler.
The super-octave may be labelled, for example, Swell to Great 4′; in the same manner, the sub-octave may be labelled Choir to Great 16′.
The inclusion of these couplers allows for greater registrational flexibility and color.
Some literature (particularly romantic literature from France) calls explicitly for "octaves aigües" (super-couplers) to add brightness, or "octaves graves" (sub-couplers) to add gravity.
Some organs feature extended ranks to accommodate the top and bottom octaves when the super- and sub-couplers are engaged (see the discussion under "Unification and extension").
In a similar vein are unison off couplers, which act to "turn off" the stops of a division on its own keyboard.
For example, a coupler labelled "Great unison off" would keep the stops of the Great division from sounding, even if they were pulled.
Unison off couplers can be used in combination with super- and sub-couplers to create complex registrations that would otherwise not be possible.
In addition, the unison off couplers can be used with the standard couplers to change the order of the manuals at the console: engaging the Great to Choir and Choir to Great couplers along with the Great unison off and Choir unison off couplers would have the effect of moving the Great to the bottom manual and the Choir to the middle manual. 
A prism coupler is a prism designed to couple a substantial fraction of the power contained in a beam of light (e.g., a laser beam) into a thin film to be used as a waveguide without the need for precision polishing of the edge of the film, without the need for sub-micrometer alignment precision of the beam and the edge of the film, and without the need for matching the numerical aperture of the beam to the film.
Using a prism coupler, a beam coupled into a thin film can have a diameter hundreds of times the thickness of the film.
Invention of the coupler contributed to the initiation of a field of study known as integrated optics.