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Which is the wrong side of the fabric?
Which side of the interfacing has the heat release glue? This will be shinier than the ‘unsticky’ side it may even have a bumpy texture which are the spots of glue waiting to be released.
Pictured is a heavy weight fusible interfacing with some pure wool tweed. Normally a heavy weight interfacing WOULDN’T be used in garment making as it has a tendency to make the fabric too ‘cardboardey’. However, if I was making myself a lovely bag out of this tweed I certainly would use this weight of interfacing as I would require the firmness it provides.