Single high E electric guitar string (9 or 10 gauge)
As any experienced guitarist will know, the high E string is the most likely string to snap (at the most inconvenient time possible, usually). It's always advisable to have a spare stash of high E strings for when this happens.
These strings are sized as 0.009 inches (9 gauge), or 0.010 inches (10 gauge).
Please note they are not long enough for Bigsby equipped guitars.