Are E26 and E27 light bulb bases interchangeable?

Yes. The E26 is the standard 120 Volt American base. The E27 is the European variant and is rated at 220 Volts. E26 is 26 mm and the E27 is 27 mm diameter. However, an E26 bulb can fit in E27 base and an E27 bulb can fit in E26 base without problem. The sockets / bulbs are interchangeable except for the voltage rating. Therefore, LED E26 bulbs that are universal line-voltage can be used in both E26 and E27 sockets.