| From the introduction, each row and each column of the 6x6 Word Square
contains the characters PLANE and a blank cell in some order, so that no
character or space can be repeated in a row or a column.  By clue 7, the
1st letter in both rows 3 and 4 is A; there are two possible arrangements
given this information: 1) A is in row 3, column 1 and row 4, column 2,
with a space in row 4, column 1; or 2) A is in row 3, column 2 and row 4,
column 1, with a space in row 3, column 1.  Using both these arrangements,
we will recover more of the letters in the Word Square.  By clue 1, we have
L then P down column 1.  If L were in row 5 and P in row 6, clue 9 conflicts.
So, L is in column 1, row 1 and P in column 1, row 2.  Then N is in column 1,
row 6 (clue 4) and E in column 1, row 5.  By clue 11, P is 1st down column 4.
P cannot be in row 2 of column 4, with space in row 1, since we already have
a P in row 2; so P is in column 4, row 1.  E is the 2nd letter down column 2
(3), so in arrangement 2) E would be in column 2, row 2.  Since E isn't the
2nd letter in row 3 (12), E would also have to be in row 2 of arrangement 1).
In both cases, there would be a letter rather than a space in column 2, row 1.
Since the P and L are already used in row 1 and the A and E are already in
column 2, N must be in column 2, row 1.  By clue 2, A is the 4th letter down
column 4 and must be in row 5.  E is the 5th letter in row 1 (8) and is thus
in column 5 or 6; by clue 10, E in column 5 can't be in row 1; E is in
column 6, row 1.  Then by clue 13, in column 6 space is in row 5 and A in
row 6.  By clue 10, P is followed immediately by E down column 5; P is in
row 3 or 5.  However, if P is in row 3 and E in row 4 of column 5, the P
is also in row 4 of column 4 (5)--no, since there is already a P in column 4.
So, P is in column 5, row 5 and E in column 5, row 6.  In row 5, N must be
in column 3 and L in column 2.  Since A is the 4th letter down column 4 (2),
column 4, row 6 must contain a letter--L is the only one that doesn't
appear in column 4 or row 6.  We now will now further test arrangements
1) and 2).  Trying 1), with A in clue 7 in row 3, column 1 and row 4,
column 2, with a space in row 4, column 1, by clue 5 in row 4 P would be
in column 3 and E in column 4.  By elimination, E in row 3 would be in
column 3.  Since E isn't the 2nd letter across row 3 (12), column 2 of
row 3 would have P in it, with the column 2 space in row 6.  However, P
in row 6 would be in column 3, where we would already have a P.  Therefore,
arrangement 1) fails.  So, by clue 7, A is in row 3, column 2 and row 4,
column 1, with a space in row 3, column 1.  By clue 12, E isn't the 2nd
letter across row 3; E must be in column 4 of row 3 and by elimination
in column 3, row 4--preceded by P in column 2, row 4 (5).  The column 2
space is in row 6, with P in row 6 in column 3.  By elimination, the P
in column 6 is in row 3; then in row 3, L must be column 2 and N in
column 5.  By clue 6, L is 4th in row 4 and must be in column 5.  Then
L in column 6 is in row 2 with N in row 4.  The row 4 space is in
column 4, with column 4, row 2 being N.  By clue 14, space in column 3
is in row 1, with A in row 2.  Finally, in column 5, A is in row 1 and
space in row 2. In sum, Word Square 7 is as follows:
 
 
  
 
|   | 1 | 2 | 3 | 4 | 5 | 6 |  
| 1  | L |      N |        | P |      A |      E |       
| 2  | P |      E |      A |      N |        | L |       
| 3  |   | A |      L |      E |      N |      P |       
| 4  | A |      P |      E |        | L |      N |       
| 5  | E |      L |      N |      A |      P |        |  
| 6  | N |        | P |      L |      E |      A |       
 
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