[abcd][abcd][abcd][abcd] \displaystyle {\displaystyle{a\over b}\brack \displaystyle{c\over d}} \textstyle {\displaystyle{a\over b}\brack \displaystyle{c\over d}} \scriptstyle {\displaystyle{a\over b}\brack \displaystyle{c\over d}} \scriptscriptstyle {\displaystyle{a\over b}\brack \displaystyle{c\over d}}