<div class="container> ... </div> 
.container {
  margin-right: auto;
  margin-left: auto;
  padding-left: 15px;
  padding-right: 15px;
  lg 1200px > +∞ --> width: 1170px; (ДЕСТОПЫ, НОУТБУКИ)
  md 992-1199px  --> width: 970px;  (СТАРЫЕ ДЕСКТОПЫ и НОУТБУКИ)
  sm 768-991px   --> width: 750px;  (ПЛАНШЕТЫ)
  xs 767px < 0   --> width: auto;   (ТЕЛЕФОНЫ)
}

  /* lg Large "TV-SET" (1200px - +infinity): */
  @media (min-width: 1200px) {...}

  /* md Medium "DESKTOP" (992px - 1199px): */
  @media (min-width: 992px) and (max-width: 1199px) {...}

  /* sm Small "TABLET" (768px - 991px): */
  @media (min-width: 768px) and (max-width: 991px) {...}

  /* xs Extra-small "MOBILE" (0 - 767px): */
  @media (max-width: 767px) {...}

LESS-переменные
Для breakpoints (@media - min-width):
  lg --> @screen-lg: 1200px;
  md --> @screen-md: 992px;
  sm --> @screen-sm: 768px;
  
Для .container (fix width):
  lg --> @container-large-desktop: 1140px + @grid-gutter-width;
  md --> @container-desktop:        940px + @grid-gutter-width;
  sm --> @container-tablet:         720px + @grid-gutter-width;
padding контейнера и колонок (0 15px):
  @grid-gutter-width: 30px;