>>>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 : )