Is a Backup Instrument Deductible if It's Rarely Used?

Yes — a backup instrument is deductible even if it's used infrequently, since the IRS doesn't require equipment to be in constant use to qualify as a legitimate business asset, only that it's kept and used primarily for business purposes. Musicians commonly keep a backup instrument specifically in case the primary one fails, needs repair, or gets damaged mid-tour — this is a widely recognized professional practice, not a personal indulgence. The fact that it might sit in its case for months...