The Company has two classes of common stock: class A common stock and class B common stock. Holders of class A common stock generally have the same rights, including rights to dividends, as holders of class B common stock, except that holders of class A common stock have one vote per share while holders of class B common stock have 10 votes per share. Each share of class B common stock is convertible at any time, at the option of the holder, into one share of class A common stock. As such, basic and fully diluted earnings per share for class A common stock and for class B common stock are the same. The Company has never declared or paid any cash dividends on either class A or class B common stock. As of March 31, 2021 and December 31, 2020, there were no shares of preferred stock issued or outstanding.

Potential shares of common stock are included in the diluted earnings per share calculation when dilutive. Potential shares of class A common stock issuable upon the exercise of outstanding stock options, the vesting of outstanding restricted stock units, and in connection with the 2021 ESPP are calculated using the treasury stock method. Potential shares of class A common stock issuable upon conversion of the 2025 Notes and 2027 Notes are calculated using the if-converted method. In computing diluted earnings per share, the Company first calculates the earnings per incremental share (“EPIS”) for each class of potential common shares and ranks the classes from the most dilutive (i.e., lowest EPIS) to the least dilutive (i.e., highest EPIS). Basic earnings per share is then adjusted for the effect of each class of shares, in sequence and cumulatively, until a particular class no longer produces further dilution.

For the three months ended March 31, 2021 and 2020, the following weighted average shares of potential class A common stock were excluded from the diluted earnings per share calculation because their impact would have been anti-dilutive (in thousands):

	Three Months Ended
	March 31,
	2021		2020
Stock Options		1,211		1,073
Restricted Stock Units		81		0
Employee Stock Purchase Plan		3		0
2025 Notes		1,633		0
2027 Notes		334		0
Total		3,262		1,073