  We consider tilings of deficient rectangles by the set $\mathcal{T}_4$ of
ribbon $L$-tetrominoes. A tiling exists iff the rectangle is a square of odd
side. The missing cell is on the main NW--SE diagonal, in an odd position if
the square is $(4m+1)\times (4m+1)$ and in an even position for $(4m+3)\times
(4m+3)$. The majority of the tiles in a tiling are paired and each pair tiles a
$2\times 4$ rectangle. The tiles in an irregular position and the missing cell
form a NW--SE diagonal crack, located in a thin region symmetric about the
diagonal, made out of $3\times 3$ squares that overlap over one of the corner
cells. The crack divides the square in two equal area parts. The number of
tilings of a $(4m+1)\times (4m+1)$ deficient square is equal to the number of
tilings by dominoes of a $2m\times 2m$ square. The number of tilings of a
$(4m+3)\times (4m+3)$ deficient square is twice the number of tilings by
dominoes of a $(2m+1)\times (2m+1)$ deficient square, with missing cell placed
on the main diagonal. If an extra $2\times 2$ tile is added to $\mathcal{T}_4$,
we call the new tile set $\mathcal{T}_4^+$. A tiling of a deficient rectangle
by $\mathcal{T}_4^+$ exists iff the rectangle is a square of odd side. The
missing cell is on the main NW--SE diagonal, in an odd position if the square
is $(4m+1)\times (4m+1)$ and in an even position for $(4m+3)\times (4m+3)$. The
majority of the tiles in a tiling are either paired tetrominoes and each pair
tiles a $2\times 4$ rectangle, or are $2\times 2$ squares. The tiles in an
irregular position and the missing cell form a NW--SE diagonal crack, located
in a thin region symmetric about the diagonal, made out of $3\times 3$ squares
that overlap over one of the corner cells.
