The T27 and T26 lamps are  interchangeable, but have a different filament type, making one more suitable than the other in certain circumstances.

The T26 has all the filaments on the same plane whereas the T27 has them running either side of one plane, creating a more compact source of light.

For use in Fresnels , there is little noticeable difference but for use in Profiles, particularly when using gobos, the T27 man give better performance.