| Grid System | It is based on 4 tier grid system (xs, sm, md, lg). | It is based on 5 tier grid system (xs, sm, md, lg, xl). |
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| CSS File | LESS | SASS |
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| Button Size | .btn-xs class is supported. | In which there is only .btn-sm and .btn-lg is available. |
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| Horizontal Form | .row class is not required using grid in forms. | .row class is required when using grid in form. |
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| Inverse Table | Not supported. | In which we can add inverse table with the help of .table-inverse class. |
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| Primary Unit | px | rem |
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| Font Size | 14px | 16px |
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| Responsive Table | In Bootstrap 3, .responsive-table class added to <div> element. | In bootstrap 4, .responsive-table class added using <table> element. |
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| Condensed Table | It support .table-condensed. | It support .table-sm. |
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| Responsive Image | Use .img-responsive class. | Use .img-fluid class. |
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| Image Alignment | Use .pull-right and .pull-left class. | You can use .pull-right, .pull-left and various other classes like .text-left and .text-center. |
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| Structure | For applying dropdown list, we use <ul>, <li> | For applying .dropdown-item we use <a>, <button>. |
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| Color | Limited colors are available it support inverse navbars but not other classes. | There are many colors option .bg-dass or .navbar-light, .navbar-dark classes. |
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| Jumbotron | .jumbotron-fluid class is not required on full-width. | .jumbotron-fluid class for full-width jumbotron. |
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| Label Pill | .label-pill is not available. | .label-pill class for making the corner round. |
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| Pager | It uses .next or .previous. | In which we use .pager-next or .pager-previous. |
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| Class | Use .breadcrumb class against the <ul> tag. | Use .breadcrumb class against the <li> tag. |
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| Afflix | Yes | No |
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| Flexbox | No | Yes |
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| Carousel Item | It uses .item class. | It uses .carousel-item class. |
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| Dividers | Apply .divider class to <li> element. | Apply .dropdown-divider class to <div> element. |
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| Table header | Does not support table header. | .thread-default and .thread-inverse are supported. |
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| Media objects | .media,. media-body, .media-heading and so on are supported. | .media is only supported. |
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| CSS unit | Bootstrap 3 supports Px as CSS unit | Bootstrap 4 supports rem as CSS unit |
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