  Let $\mathcal{H}$ be a separable Hilbert space and $P$ be an idempotent on
$\mathcal{H}.$ We denote by $$\Gamma_{P}=\{J: J=J^{\ast}=J^{-1} \hbox{ }\hbox{
and }\hbox{ } JPJ=I-P\}$$ and $$\Delta_{P}=\{J: J=J^{\ast}=J^{-1} \hbox{
}\hbox{ and }\hbox{ } JPJ=I-P^*\}.$$ In this paper, we first get that
symmetries $(2P-I)|2P-I|^{-1}$ and $(P+P^{*}-I)|P+P^{*}-I|^{-1}$ are the same.
Then we show that $\Gamma_{P}\neq\emptyset$ if and only if
$\Delta_{P}\neq\emptyset.$ Also, the specific structures of all symmetries
$J\in\Gamma_{P}$ and $J\in\Delta_{P} $ are established, respectively. Moreover,
  we prove that $J\in\Delta_{P}$ if and only if
$\sqrt{-1}J(2P-I)|2P-I|^{-1}\in\Gamma_{P}.$
