For now we are using 4 breakpoints for 4 different type of screen, it's the default breakpoints of bootstrap framework. Bootstrap framework is mobile-first based, therefore we will not violate this concept.

• Extra small devices - Phones (<768px)
• Small devices - Tablets (≥768px)
• Medium devices - Desktops (≥992px)
• Large devices - Desktops (≥1200px)

Documentation: http://getbootstrap.com/css/#responsive-utilities

In LESS files we can use this breakpoints as variables together with media-queries.

• Extra small devices -> @screen-xs
• Small devices -> @screen-sm-min
• Medium devices -> @screen-md-min
• Large devices -> @screen-lg-min

We should define all common styles for any element for without any breakpoint (by default it's will be applied for all screen) and we must use media-queries for override or add custom properties to selector.

For example, our-block.less:

/* it will be applied for all screen sizes, it will be block 100% width and 100% height parent block and have red background  */
.our-block {
	display: block;
	width: 100%;
	height: 100%;
	background-color: red;
}
/* it will be applied to all screen starting from small devices where screen width ≥ 768px */
@media (min-width: @screen-sm-min) {
.our-block {
	width: 150px;
	height: 150px;
}
}

/* if we want to set property only for one breakpoint we should set up min-width and max-width */
@media (min-width: @screen-sm-min) and (max-width: @screen-sm-max) {
.our-block {
	width: 200px;
	height: 200px;
}
}
/* in result we will have 200x200 only on screens where width ≥ 768 < 992 for rest (medium, large) we have previous values (150x150) */