Heterostructures are artificially fabricated layer structures made of two or more different substances [1].  Most frequent are combinations AlAs with GaAs (or AlGaAs), but other III-V and II-VI compounds, Si and Ge are used as well.  Porous silicon layers with modulated porosity have also been studied.  In quasiregular heterostructures, as distinct from periodic “superlattices”, the layer sequence is governed by some aperiodic algorithm.  Most of these algorithms are based on substitution sequences [2].  These can also be interpreted as automatic sequences.  To exploit the full richness of quasiregular heterostructures it is expedient to study double-sided sequences.  They are also a suitable starting point for multidimensional generalizations.  I present here simple ways to construct double-sided sequences and finite double-sided chains.  Specifically, I deal with the double-sided versions of the most popular sequences: Fibonacci, Prouhet-Thue-Morse, paperfolding, period doubling and Golay-Rudin-Shapiro.  Their spectral properties are the same as of their single-sided counterparts.
[1]  García-Moliner F., Microelectronics Journal 26 (2005) 870-875.
[2]  On-Line Encyclopedia of Integer Sequences,
       http://www.research.att.com/~njas/sequences/Seis.html