Modal verbs

Uses of should

In the following sentence should has the meaning of in the case that.

• It's been thought that when you have more friends than the magic 150, these are backup friends – contacts you might call on should your other friendships fall away.

Other uses of should are as follows:

• Here is a map should you get lost. (meaning "in the event that") 
• You should wear a seat belt. (advice/obligation) 
• She should be back today. (deduction)  
• I should have got up earlier. I missed the plane. (perfect infinitive for past events that did not happen)
• If you should see Jane, tell her the news. (if-clauses)
• I'll make more dinner in case Helga should turn up. (after in case)
• I turned out the light so that it shouldn't keep Amy awake. (after so that)
• It's incredible that he should say such a thing. (judgment or reaction)