The definition for 'extensible' at m-w.com reads 'capable of being extended'. So what's 'extendable' useful for? - it says it means 'to stretch'. I was surprised recently to find that XML is 'extensible markup language', not 'extendable' as I previously thought. 'Extensible' seems therefore to mean something that can be added to like XML, whereas 'extendable' would apply to something more like a telescopic lens, but there doesn't seem to be a very solid convention here - or is there? Which do you guys favour and for which usages?