When I was a teenager, my father had multiple strokes that affected his speech. At his best after the stroke, he would respond to conversation but rarely initiate it. While my mother worked, my siblings and I would take turns caring for him. My father was an engineer by education (and to the core). We used to engage in dialogues to have some type of conversation. I would tell him that I loved him, he would respond I love you more, back and forth...until he responded "I love you TAN 90" (full disclosure, I am an engineer too) to which I would respond, "Daddy, that is 'undefined' because you are dividing by zero." His final words were, "No, it is infinity."
When I met my husband and fell in love (also an engineer), I told him the story and he embraced it. We both used to exchange our "I love you's" with "TAN 90" instead. I would end my notes, cards, and letters with "tan 90" vs. 'Love, Caroline.'
We have been married for 26 years now and have 4 children. As my children have learned trigonometry and understood the mathematical expression, they have grown to appreciate what I meant when I used "TAN 90" on them over their lifetimes.
My oldest daughter bought me my key and had TAN 90 stamped on it to underscore her appreciation for our relationship, for me, for her grandfather (whom she never met), and my legacy. I treasure my key and love to tell the story.
I hope this makes sense and touches another person as well. TAN 90, Daddy xoxo.