Thank you for the explanation.

>
>You can't ignore the . - that's a CSS directive that tells the CSS parser that the **declaration** is a class.
>
>. = CSS class
>[] = Element Attribute
># = ID
>no prefix = Element
>
>for example.
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>The last item is key - because elements don't take a prefix they are a special case.
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>So in CSS:
>
>

>div {   ... } 
>.my-class { ... }
>*[data-id]  { ... }
>#MyControl
>

>
>
>All select the following element:
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>

><div id="MyControl"   data-id="3421" class="myclass someotherclass" >Content</div>
>

>
>
>As to CSS classes: You define them with a `.` but you use them without.
>
>+++ Rick ---
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>
>
>
>>>>Hi,
>>>>When I read Bootstrap 4 documentation, very often a class name has a prefix period (.). For example, .btn-group-toggle or .d-none and so on. What is the meaning of the period (.)? When I use classes in the HTML5 code, I don't think I enter them. For example, I could say