LtRt is a L C R S ( left center right and mono surround ) mix that is encoded down to a , and i’m going to use this very loosely, “stereo” mix. The “stereo” mix is really the four tracks encoded to 2 tracks. Then upon playback the LtRt can be heard in just plain mono or stereo or decoded back to L C R S. LtRt or Dolby Surround was the first of Dolby’s multichannel film format. ( 1980 something ) The encoded file fits nicely on VHS or other 2 track device and then you get surround off of these 2 track systems.
The encoding process relies heavily on phase to encode and decode the positional information within the mix.
you can use a DOLBY SEU 4 or Dolby DP563 hardware devices to encode LtRt, or you can create an LtRt optical track starting with a Dolby DMU or LtRt files… You can use the DOLBY surround tools in protools to create LtRt ( both encode and decode )…
remember that an LtRt track is just analog audio vs an AC3 or Dolby-E encode which is a digital bit stream. You can create an AC3 encode using only L C R S very similar to an LtRt for DVDs with A-Pack or other AC3 encoder, but again it will create a digital bit stream, not an analog audio file.
to mix… just mix in L C R S… paying close attention to any coupling or phase issues with similar materials in multiple speakers… music, ambience etc. then downmix to LtRt and LoRo and mono checking each for positing of material. When mixing you can use the Dolby Surround tools to create 2 stereo aux tracks with the encoder on them Master( L,R ) slave( C,S ) and them place the decoder (master, slave) after the Encoder so that you can monitor thru the encoder/decoder and simply hit a button to hear whats going on in LCRS, LR, mono… the details will become obvious when you look at the software plugins…
cheers
geo
PS: i think today the whole this is called DOLBY ANALOG by Dolby… sorry I haven’t had my morning coffee so this reply is a little disjointed.. 
cheers
geo
some additional from Neil Wilkes across the pond…
There is Dolby ProLogic and Dolby ProLogic II, which are utterly different systems despite the incremental naming protocol – DPL has dual mono rear channels & feeds the same information to both Ls & Rs and DPL II uses proper separate Ls/Rs where available. In addition, all DPL II en/decoders are fully backwards compatible with the earlier systems.
SRS Circle Surround is also a Matrix Lt/Rt technology.
Lt/Rt is basically a generic term taken to mean any multichannel mix that is matrixed down into a stereo compliant stream & will play back on a stereo system, but if the stream is fed through the correct decoder you will get the surround mix back out of it.
There are Pros & Cons to this.
Pros -
1 – you get to supply a single stream that will play back in stereo if no surround setup is present.
2 – It is widely used in TV/Broadcast.
3 – You can even take the Lt/Rt stream & further reduce to AC3 (Dolby Digital) as long as you set the metadata flag (in a soft encoder) or push the switch/button (in a hardware encoder) for “Dolby Surround Encoded”
Cons -
1 – It is a compromise for both types. If you start with a fully discrete 5.1 mix, then it will *not* sound the same when decoded again. Neither will the stereo mix be as good as one that was especially mixed for stereo.
2 – It’s a matrix system – not discrete – and as Georgia points out there are all manner of possible pitfalls involved.
3 – It makes delivering an M&E mix much more complex as you would need to completely reset the encoder resulting in a vastly different sounding mix on a dubbed language version.
When I have to deliver an Lt/Rt stream I use a VST plugin en/decoder so that switching between the various modes is extremely easy (source/encoded/decoded).
A good place to start research on this is at the Dolby Labs technical library where you will find a lot of helpful material.
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