Depreciation rules on block assets, concessional tax regimes, and special 40% allowance for eligible technology-based machinery. Depreciation under Rule 5 is allowed on the written down value of a block of assets at the percentages in Appendix I, subject to a forty per cent cap for specified taxpayers who have exercised concessional tax options. The rule also adjusts written down value where earlier depreciation or unabsorbed depreciation attributable to section 32(1)(iia) is not set off, prescribes depreciation on actual cost for certain assets under Appendix IA, and permits a qualifying power-generating undertaking to opt for the general depreciation rule. A special forty per cent rate applies to specified new machinery or plant using technology or know-how developed in recognised laboratories, subject to certification and other conditions.