The most obvious answer to 
the question of what makes 
one figure look like another is 
that it is the number of parts, 
or "features," they have in 
common. Some letters of the 
alphabet, for instance, have 
common features: both O and Q 
contain an oval, both M and W 
have four vertical or near 
vertical lines, an E is an F 
with an extra line, and so on. 
 
    However, the geometry of a 
figure—–its shape—–can hardly 
be reduced to a sum of its parts. 
Consider the figures to the left. 
If the parts of an E are 
rearranged, there may be little 
similarity between it and an E. 
Clearly it is the position of the 
parts relative to one another 
that matters. If the relations 
between the parts are 
preserved, as is the case in 
each of these variations, the 
whole shape will remain 
unchanged, despite rather 
extreme changes in the parts 
themselves. This was a point 
the Gestaltists emphasized: 
Visual and auditory patterns 
depend upon stimulus 
relations; if such relations are 
preserved, a pattern can be 
transposed without any effect 
on the perceived similarity of 
the whole. In music, one can 
transpose a melody in key or 
octave and preserve the 
recognizability of the melody. 
All or many of the parts, the 
individual tones, are now 
different from those in the 
original melody, but the tonal 
"steps" and rhythmical 
relations are preserved. By the 
same token, a visual pattern 
can be transposed in size, in 
the make-up of the contour, 
and in the location of the 
image on the retina without 
altering its appearance very 
much, if at all.