THE LEAN-TO, 11 
in the majority of cases, as the plants within receive more 
light, and this reaches them more equally on all sides 
than in either of the other forms, while such houses may 
be erected in any open position away from an existing 
wall or building. For both the other forms a back wall 
of some kind is necessary. Where this is under 8ft. or 9ft. 
in height the three-quarter span form is most suitable, 
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but if the wall is much higher than this a lean-to structure 
had better be erected. Upon the dimensions and 
proportions of the house, however, the proper height of 
the back wall depends, of course, in all cases. Where 
there is already an existing back wall, equally suitable for 
either form of structure, the three-quarter span should, as 
a rule, receive the preference, as even when the front 
