How do I use the Spacing Utility Classes on Bootstrap 4

Update 2020 Bootstrap 4

The Bootstrap 4 spacing utils have changed since the original answer (for v4 alpha). Now the syntax is simply:

• margins: m{sides}-{size}
• padding: p{sides}-{size}

Examples..

mb-2 = margin bottom 2 spacing units
m-0 = no margins
pt-3 = padding top 3 spacing units
p-1 = padding all sides 1 spacing unit

There are now 6 sizes (0-5) that represent a portion of the standard .5rem spacer unit. Additionally, x-axis (left/right) and y-axis (top/bottom) utils have been added:

my-2 = margin top & bottom 2 spacing units
mx-0 = no margins left & right
px-3 = padding left & right 3 spacing units

And, the spacing utils are now responsive. The smallest xs breakpoint is implied when no breakpoint is used.

• margins: m{sides}-{breakpoint}-{size}
• padding: p{sides}-{breakpoint}-{size}

mt-md-2 = margin top 2 spacing units, on md (and up)
py-sm-0 = no padding top & bottom, on sm (and up)

Bootstrap 4 Spacing Utils Demo

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Related: Bootstrap 4 spacing bug?

Refer to the Spacing (Bootstrap v4 alpha) documentation.

The classes are named using the format: {property}-{sides}-{size}

Where size is one of: * 0 - for classes that eliminate the margin or padding by setting it to 0 * 1 - (by default) for classes that set the margin or padding to $spacer-xor $spacer-y * 2 - (by default) for classes that set the margin or padding to $spacer-x * 1.5 or $spacer-y * 1.5 * 3 - (by default) for classes that set the margin or padding to $spacer-x * 3 or $spacer-y * 3.

So use m-t-3 instead of m-t-lg.

Basically its only works from 0 to 3. m-t-0, m-t-1, m-t-2, or m-t-3