# Math mode: Why \left and \right is considered bad? [duplicate]

A most respected moderator repeatedly advises against the automation of \left and \right in math mode. (Among others, here is one.) There must be some good argument behind that. I am just curious, why?
## marked as duplicate by Mico math-mode StackExchange.ready(function() { if (StackExchange.options.isMobile) return; $('.dupe-hammer-message-hover:not(.hover-bound)').each(function() { var$hover = $(this).addClass('hover-bound'),$msg = $hover.siblings('.dupe-hammer-message');$hover.hover( function() { $hover.showInfoMessage('', { messageElement:$msg.clone().show(), transient: false, position: { my: 'bottom left', at: 'top center', offsetTop: -7 }, dismissable: false, relativeToBody: true }); }, function() { StackExchange.helpers.removeMessages(); } ); }); }); Aug 14 '17 at 14:14
• For one thing, \left...\right cannot break across lines, whereas \big etc. have no such limitations. I would guess the other argument is that one should deliberately choose their delimiter sizes, rather than trusting them to an algorithm. – Steven B. Segletes Aug 14 '17 at 13:59
• Try $a(b+c)$ and $a\left(b+c\right)$ – egreg Aug 14 '17 at 14:00