the spousal IRA rule is  - provided a joint return is filed, the lower-earning spouse (even if no earned income) can consider the other spouse's earned income to the extent it has not been taken into account in making the IRA contribution for the higher-earning spouse.

so if your earned income is only $4000, and you contribute $3000 to your IRA you can only contribute $1000 to your spouse's IRA whereas if your earned income is $20,000 you can max out both.