Here are some possibilities:
Music:
* "Strike up the band!" This refers to starting the music, which involves horizontal motion of the conductor's baton or the musicians' instruments.
Business:
* "Strike up a deal." This refers to initiating a business transaction, which is neither strictly vertical nor horizontal. It involves negotiation and agreement, which can be represented by moving along a metaphorical line.
Fire:
* "Strike up a fire." This involves vertical motion, as you would strike a match or lighter downwards towards the tinder.
Other:
* "Strike up a conversation." This is not a physical movement and is therefore neither vertical nor horizontal. It involves the act of starting a conversation.
Can you please provide more context so I can give you a more accurate answer? For example, what is the specific situation you are asking about?
