  Double Boolean algebras are algebras
$\underline{D}=(D;\sqcap,\sqcup,\neg,\lrcorner,\bot,\top)$ of type
$(2,2,1,1,0,0)$ introduced by Rudolf Wille to capture the equational theory of
the algebra of protoconcepts. Every double Boolean algebra $\underline{D}$
contains two Boolean algebras denoted by $\underline{D}_{\sqcap}$ and
$\underline{D}_{\sqcup}$. A double Boolean algebra $\underline{D}$ is said pure
if $D=D_{\sqcap}\cup D_{\sqcup}$, and trivial if
$\bot\sqcup\bot=\top\sqcap\top$. In this work, we first show that a double
Boolean algebra is pure and trivial if and only if it is a glued sum of two
Boolean algebras; secondly, we characterize simple double Boolean algebras; and
finally, we determine up to isomorphism all sub-directly irreducible algebras
of some sub-classes of the variety of double Boolean algebras.
