\PGset[0.8em]
\begin{picture}(20,10)

\drawline(1,1)(12.3,1)(12.3,9.4)(1,9.4)(1,1) % R
\drawline(16,1)(19,1)(19,4)(16,4)(16,1) % U

\put( 6.2, 5.0){$\scriptstyle R$}
\put( 0.3, 5.1){$\scriptstyle a$}
\put( 6.4, 0.3){$\scriptstyle b$}

\put(17.2, 2.2){$\scriptstyle U$}
\put(15.3, 2.2){$\scriptstyle 1$}
\put(17.3, 0.3){$\scriptstyle 1$}

\end{picture}
\PGrestore