Started in 1989? Why didn't I hear about TAD for so long?
Good question. There are many reasons. There are some intricate tech reasons why works like TAD sometimes are not exposed through Google, etc.

There are two nice TED talks on the Google filter bubble:

• https://www.ted.com/talks/andreas_ekstrom_the_moral_bias_behind_your_search_results and
• https://www.ted.com/talks/eli_pariser_beware_online_filter_bubbles

There were some personal events that also hindered the exposure. But that is all now past and quite irrelevant as an answer here.

But one main reason is: this kind of approach requires a lot of vetting. Sometimes, deductive logic is insufficient to justify an approach. You need inductive logic also to explain why an approach like TAD is valid.

Sabu Francis was a young man – just 28 years old– when he discovered this approach … Those days, people had lot of skepticism. “Is this really useful in all kinds of buildings?” “Aren't you too inexperienced?”

It seemed valid to ask that question. Nobody seemed to have the time to look at the deductive thinking behind the math inside TAD. People wanted examples of its usage. I guess it requires a passage of time, lots of actual projects done using TAD, two research projects with a leading institute in India (IIT Bombay), theoretical papers in conferences… and fair amount of gray hair (and the loss of it) before that work of a young man can really be considered.

I hope the reader of this answer would also consider using TAD.

TAD was finally “exposed” to the www on Jan 26, 2017, after a conference on computing in architecture at a college of architecture in Ooty, India.

You say it is free. Any catch?
Not at all. We don't even lock you in to buying anything at all from us, if that is how you work – the software is completely open and you can create your own architectural masterpieces and make real buildings with them.

Our liberal freeware license indicates that if you happen to sell your data to potentially anonymous users, then it has to be routed through our payment gateway and our servers. We thought it is only fair that we make money only when you make money – and that too for data that goes to any potentially anonymous user

The entire TAD package would be having some commercial offerings too (like the architectsdesktop private cloud servers which you can purchase if you so want) but we are committed to make none of them mandatory as such. We would also offer advice on a subscription basis, and other commercial services too

We want to be completely transparent. We believe it is a fair deal. If you have more suggestions, we would gladly listen and implement the nice ones.

Is there any monetary incentive for users of TAD?
Certainly . TAD has a scripting system. You can create add-ons using that; and rent them to others. We will take a small slice of the rent. So we make money, when you make money too. There thousands of calculations that architects need to do – but often do not due to various reasons. For all you know you could solve a few of these calculations, and make add-ons (probes) that can be rented to others for a fee.

How can I help?
Tell all architects to use TAD! Inform on Facebook, Twitter and other social media. Let this go viral

Here is a personal appeal from Sabu Francis which you can read, if you want to help.

Does it work like Autocad or Revit or Archicad or Sketchup … ?
No. TAD does not work like any of those software. That does not mean it is hard to use. Just open your mind and read our documentation here. You should be able to learn it easily and you can start producing designs in half a day or so

But is TAD really a BIM software?
Yes it is. And no, it is not.

BIM is a term invented by Autodesk, to give a generic name to the category of software like Revit. A BIM software talks about information inside the model of a building. That information need not be just the visual 3D of the building. It would be things like cost, properties of material, name of supplier and a zillion other stuff. BIM was coined only around 2007(?) or so.

The questions that BIM addresses were being asked almost from the sixties. But BIM is lot more pithy term that allowed people to catch on fast.

Before adopting the term BIM, TAD was referred to object-oriented design software. And no TAD is not like Revit. Or Archicad or any of the IFC based BIM software. Since TAD can talk of all those non-visual properties also ; we still use the term BIM here.

It talks about information in a building way before the design has actually become a proper usable model of a building. We call it an early stage BIM software

We should be possibly be calling it AIM (Architecture Information Modeling) instead of BIM. More on that later.

What is the difference between TAD, TAD Designer and TAD Designer Lite?
TAD stands for The Architect's Desktop it is a 'mother' software containing several other software and web services bundled in it. TAD Designer Lite is the designing component that works on your computer as a standalone executable. It is one of the oldest, if not THE oldest, BIM software. Being a BIM, it not only handles a 3D model, but also a lot of information that is present in the design

TAD Designer Lite has some small geometric limitations such as not able to make a dome and other shell forms.

TAD Designer will not have any such limitations. But this is still to be written. We have also written a few other components for TAD but mostly people use TAD Designer Lite which is robust and was used since 1989.

Hence, for now we often use TAD Designer, TAD Designer Lite and TAD interchangeably.

Why is it a desktop?
The final objective of this software is simple and emphasizes the meaning of the word “desktop” in the life of professionals involved in the AEC (Architecture, Engineering Construction) Industry. TAD is a platform for professional architects to design, manage real architecture right from its conception to usage and also the working of an architect's office.

Architects and other professionals from the AEC industry should be able to start their computer and get a desktop in which EVERYTHING concerning their architectural projects can be carried out. Another important objective is that the data used in TAD would be put to use for everyone, even by non-architects, even after the architect is no longer around

Why both freeware version and open source version? What is the difference?
TAD Designer Lite versions 6.0.0.0 to 7.0.0.0 would be freeware. It is written and maintained in some old, commercial languages (Visual Prolog and Delphi) so is not suited for an open-source development system.

Very soon, we would be working on TAD Designer version 7.x onwards. That would be developed in an open-source development environment. It would have additional geometrical capabilities.

Both these i.e. TAD Designer 7.x as well as TAD Designer Lite would be free to use.

The reason for giving TAD Designer Lite free are many. This documentation explains all that idealism.

The community can actively participate and give feedback using the TAD Chat system (Registration at the TAD Chat system is mandatory. That is also free.)

As you use it, you can directly post bugs/suggestions/issues etc right from within TAD Designer Lite

You use the term Lite for TAD Designer. Does it mean that is free, and the full one is not?
No. The term Lite is only due to a few geometric modelling limitations. Currently it does not handle some curves, domes, BRep meshes. Those are all being worked on, and then the term Lite would go away from TAD Designer Lite and it would be only TAD Designer We are committed to make even TAD Designer free, not just the Lite version. As explained elsewhere, there are ways to make money both for us as well as for you. But not by selling the software to you

Will my email address and password be misused?
Absolutely not. We ask for you to register with us only for holding the entire community together. From the time we moved the teamTAD community to the publicly hosted Discord system; emails are not mandatory. However, we strongly request that you provide a valid email just in case we want to share some critical information. The email is requested of you when running the installer.

You would receive (very occasionally or one can say; rarely) emails from us. If you are super-worried about your email address and/or password; you can setup a free gmail account in five minutes and use that gmail address for participating.

There is a chat window in the documentation. What is that?
The chat inside the documentation itself is for the general public; and mainly discusses the documentation itself. You would need to have an account on Twitter or Github if you want to post into that chat.

That chat working within the documentation will NOT make you a registered user of TAD.

The process of registering yourself as a registered user of TAD is explained here