1. Items of deduction must be supported by documents 2. As a rule, deductions are allowed if they are taken from gross estate or are chargeable to gross estate 3. Double deduction is not allowed in estate taxation 4. For married decedents, deductions are presumed common unless proven to be exclusive 5. Vanishing deduction is a special deduction 6. The share of the surviving spouse is 36 of the husband's exclusive property 7. Ordinary deductions normally result in reduction in the hereditary estate 8. Special deductions do not reduce the hereditary estate of the heirs 9. Special deductions are allowed only to resident or citizens decedents 10. Non-resident decedents cannot claim the full amount of ordinary deductions 11. Obligations of the surviving spouse are deductions against gross estate 12. The deduction for share of surviving spouse does not apply to unmarried decedents 13. The estate tax liability of the decedent is deductible in the computation of the net taxable estate 14. The vanishing deduction is applicable only if he previous estate paid estate tax 15. The vanishing deduction is applicable only to properties inherited by the decedent within five years before his death