Let $Omega$ be a closed and bounded polygonal domain in R$^2$, or a closed line segment in R$^1$ with boundary $Gamma$, such that there exists an invertible mapping T : $Omega$ longrightarrow $Omega$ with the following correspondence: x$in$$Omega$ ↔ x = T(x) $in$$Omega$, (1.1) and (1.2) t $in$ U$sub$p/($Omega$) ↔ t = to T$^$-1/ $in$ U$sub$p/($Omega$), where $Omega$ denotes the corresponding reference elements I = [-1,1] and I ${ imes}$ I in R$^1$ and R$^2$ respectively, (1.3) U$sub$p/($Omega$) = {t : t is a polynomial of degree $leq$ p in each variable on $Omega$}, and (1.4) U$sub$p/($Omega$) = {t : t = to T $in$ U$sub$p/($Omega$)}.(omitted)