a lack of agreement between the taxpayer and the IRS, the IRS formally "determines" the amount of the "deficiency" and issues a formal notice called a "statutory notice of deficiency," or "ninety day letter". In this context, the term "deficiency" is a legal term of art, and is not necessarily equal to the amount of unpaid tax (although it usually is). The deficiency is generally the excess of the amount the IRS contends is the correct tax over the amount the taxpayer showed on the return—in both cases, without regard to how much has actually been paid. Upon issuance of the statutory notice of deficiency (after IRS determination of the tax amount, but before the formal IRS assessment of the tax), the taxpayer generally has 90 days to file a Tax Court petition for "redetermination of the deficiency". If no petition is timely filed, the IRS may then statutorily "assess" the tax. To "assess" the tax in this sense means to administratively and formally record the tax on the books of the United States Department of the Treasury. This formal statutory assessment is a critical act, as the statutory tax lien that later arises is effective retroactively to the date of the assessment, and encumbers all property and rights to property of the taxpayer. Life cycle of a Tax Court case Because of the negative legal consequences ensuing with respect to a statutory assessment (especially the tax lien and the Flora requirement that the taxpayer otherwise pay the full disputed amount and sue for refund), a taxpayer is often well advised to file a Tax Court petition in a timely manner. The rule in the Tax Court is that the taxpayer sues the "Commissioner of Internal Revenue," with the taxpayer as "petitioner" and the Commissioner as "respondent." This rule is an example of an exception to the general rule that the proper party defendant in a U.S. tax case filed by a taxpayer against the government is "United States of America." In the Tax Court, the Commissioner is not named personally. The "Secretary of the Treasury", the "Department of the Treasury" and the "Internal Revenue Service" are not proper parties. The petition must be timely filed within the allowable time. The Court cannot extend the time for filing which is set by statute. A $60 filing fee must be paid when the petition is filed. Once the petition is filed,