What Is Class="Mb-0" in Bootstrap 4?

Bootstrap 4

It is used to create a bottom margin of 0 (margin-bottom:0). You can see more of the new spacing utility classes here:

Related: How do I use the Spacing Utility Classes on Bootstrap 4

m - for classes that set margin, like this :

• mt - for classes that set margin-top
• mb - for classes that set margin-bottom
• ml - for classes that set margin-left
• mr - for classes that set margin-right
• mx - for classes that set both margin-left and margin-right
• my - for classes that set both margin-top and margin-bottom

Where size is one of margin :

• 0 - for classes that eliminate the margin by setting it to 0, like mt-0
• 1 - (by default) for classes that set the margin to $spacer * .25, like mt-1
• 2 - (by default) for classes that set the margin to $spacer * .5, like mt-2
• 3 - (by default) for classes that set the margin to $spacer, like mt-3
• 4 - (by default) for classes that set the margin to $spacer * 1.5, like mt-4
• 5 - (by default) for classes that set the margin to $spacer * 3, like mt-5
• auto - for classes that set the margin to auto, like mx-auto

Bootstrap has predefined classes that we use for styling. If you are familiar with CSS, you'd know what padding, margin and spacing etc. are.

mb-0 = margin-bottom:0;

OK now moving a little further in knowledge, bootstrap has more classes for margin including:

mt- = margin-top
mb- = margin-bottom
ml- = margin-left
mr- = margin-right
my- = it sets margin-left and margin-right at the same time on y axes
mX- = it sets margin-bottom and margin-top at the same time on X axes

This and much more is explained here The best way to learn is through site itself. It explains a lot obout its built in classes.

m - sets margin

p - sets padding

t - sets margin-top or padding-top

b - sets margin-bottom or padding-bottom

l - sets margin-left or padding-left

r - sets margin-right or padding-right

x - sets both padding-left and padding-right or margin-left and margin-right

y - sets both padding-top and padding-bottom or margin-top and margin-bottom

blank - sets a margin or padding on all 4 sides of the element

0 - sets margin or padding to 0

1 - sets margin or padding to .25rem (4px if font-size is 16px)

2 - sets margin or padding to .5rem (8px if font-size is 16px)

3 - sets margin or padding to 1rem (16px if font-size is 16px)

4 - sets margin or padding to 1.5rem (24px if font-size is 16px)

5 - sets margin or padding to 3rem (48px if font-size is 16px)

auto - sets margin to auto

Since this "thread" is the first one to appear when searching for class="mb-0" I'd like to mention that all these answears are deprecated. Now in February 2022 bootstrap is V5.1 and this it the link to the new spacing notation to use in your classes: (the main problem was with left and right which are now to be reference as start and end in the notation)

class="mb-0"

m - sets margin

b - sets bottom margin or padding

0 - sets 0 margin or padding

CSS class

.mb-0{
       margin-bottom: 0
     }