>>>>I am struggling with what should be a simple task: hiding a div section on displays smaller than md.
>>>>
>>>>Here is just one of a sample codes I tried
>>>>

>>>><div class="container col-lg-12 hidden-lg-down"</div>
>>>>

>>>>
>>>>What am I missing? I should I have a certain meta in the header section of the page? I have the following:
>>>>

>>>> <meta name="viewport" content="width=device-width, initial-scale=1">
>>>>

>>>>
>>>>TIA
>>>
>>>I found that the d. classes work (which I have never used before and didn't even know of their existence). The URL:
>>>
>>>https://getbootstrap.com/docs/4.0/utilities/display/
>>
>>As you discovered it all changed in 4.0.
>>If something is not working as expected in bootstrap the first thing to do is check bootstrap.css and make sure that the classes you at trying to use are actually defined :=}
>
>IMHO, this is not good (I want to use a stronger word but don't want to offend anybody at Bootstrap :)). If I were upgrade my older project (where I used Bootstrap 3) with Bootstrap 4, many things would break. I always thought that a good practice for a software is to be backwards compatible. I guess no one listens to me : )

Always a mixed blessing. If site is using bootstrap 3 then it's probably not worth migrating to 4.0. Main thing is to be sure that any documentation you are reading is for the version you are using. Situation is worse on sites like code project - Q&A's often don't mention the version in question (no pun intended).

FWIW, angular is an extreme example of this. Angular 2 was a complete rewrite of angularjs (Angular 2 is far better IMO) but I often wish that they had changed the name completely to avoid confusion.