Merging tables in R with 2 common columns in R

Question

I have 2 tables which need to be merged, based on 2 common columns. Table A:

Year	Month	Temp
2011	1	10
2011	2	20
2012	1	30

Table B:

Year	Month	Pressure
2011	1	100
2011	2	200
2012	1	300

I want a final table which includes all the 4 columns i.e. Year, Month, Temp and Pressure.