\def\half{\textstyle\frac{1}{2}}
\def\hhalf{\scriptstyle\frac{1}{2}}
\def\quarter{\textstyle\frac{1}{4}}
\def\qquarter{\scriptstyle\frac{1}{4}}
\def\squarter{\sfrac{1}{4}}
\def\stwelfth{\sfrac{1}{12}}
\def\sthirtysixth{\sfrac{1}{36}}
\def\grad{\boldsymbol{\nabla}}
\def\gradh{\boldsymbol{\nabla}_h}
\def\bgamma\boldsymbol{\gamma}
\def\rdt{\Delta t}
\newcommand{\ew}[3]{{e_{3#1}}_{\,#2}^{\,#3} }
\newcommand{\vect}[1]{ \rm{\textbf{#1}} }    % vector style: non-italic bold
\def\deg{\degres}                            % degrees  (NB: \r{} can also be used)