Buttons
Default buttons
Bootstrap includes six predefined button styles, each serving its own semantic purpose.
Outline buttons
Replace the default modifier classes with the .btn-outline-* ones to remove all background images and colors on any button.
Rounded buttons
Use class .btn-rounded for button round border.
Buttons with icon
Add icon in button.
Buttons Sizes
Add .btn-lg or .btn-sm for additional sizes.
                                
Buttons width
Add .w-xs, .w-sm, .w-md and  .w-lg class for additional buttons width.
                                
Button tags
The .btn
                                    classes are designed to be used with the <button> element.
                                    However, you can also use these classes on <a> or <input> elements (though
                                    some browsers may apply a slightly different rendering).
Toggle states
Add data-toggle="button"
                                    to toggle a button’s active
                                    state. If you’re pre-toggling a button, you must manually add the .active class
                                    and aria-pressed="true" to the
                                    <button>.
                                
Block Buttons
Create block level buttons—those that
                                    span the full width of a parent—by adding .d-grid.
Checkbox & Radio Buttons
Create button-like checkboxes and radio buttons by using .btn styles rather than .form-check-label on the <label> elements.
Button group
Wrap a series of buttons with .btn in .btn-group.
Button toolbar
Combine sets of button groups into button toolbars for more complex components. Use utility classes as needed to space out groups, buttons, and more.
Sizing
Instead of applying button sizing
                                    classes to every button in a group, just add .btn-group-* to each .btn-group, including each one
                                    when nesting multiple groups.
Vertical variation
Make a set of buttons appear vertically stacked rather than horizontally. Split button dropdowns are not supported here.