Day Count Basis
Indicates the manner in which the days in both the month and the year are to be counted for interest payments and accruals, as well as yield-to-maturity calculations.  There are four basic types of day count basis used and generally the security type is the determining factor.  The convention for indicating day count basis is days in the month/days in the year.

Actual/Actual --- assumes the year has either 365 or 366 (leap year) days
30/360 --- assumes all months have 30 days and the year has 360 (12x30)
Actual/360 --- assumes the year has 360 days
Actual/365 --- assumes the year has 365 days