Book income is determined using accrual accounting. However, taxable income may be determined using accrual, cash-basis, or a hybrid method, provided the method is used consistently and accurately reflects income. The book and tax treatment of specific items of income and expense also differ; some differences are permanent, while others relate to timing. For example, each system may depreciate assets over a different number of years. Some costs may be deductible in calculating book income while they may be allowed as credits or deductions for tax purposes. All meals and entertainment expenses are deductible in arriving at book income, but tax rules allow a deduction for only 50% of business meals and no deduction at all for business entertainment. Tax-exempt interest is excluded from taxable income but is included in book income. Fines and penalties reduce book income but are not deductible under tax rules. Some tax treatments are elective, but financial accounting rules that determine book income are standard.