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But what about the first pre-treatment period (1900 in your example)? Assuming a placebo treatment (1 for observations that are in treatment group and year is 1900), every observation in the treatment group is coded with one and then collinear with the individual fixed effects, leading the first pre-treatment interaction to be dropped.

But what about the first pre-treatment period (1900 in your example)? Assuming a placebo treatment (1 for observations that are in treatment group and year is 1900), every observation in the treatment group is coded with one and then there is multicollinearity, leading the first pre-treatment interaction variable to be dropped.

But what about the first pre-treatment period (1900 in your example)? Assuming a placebo treatment (1 for observations that are in treatment group and year is 1900), every observation in the treatment group is codes with one and then collinear with the individual fixed effects, leading the first pre-treatment interaction to be dropped.

But what about the first pre-treatment period (1900 in your example)? Assuming a placebo treatment (1 for observations that are in treatment group and year is 1900), every observation in the treatment group is coded with one and then collinear with the individual fixed effects, leading the first pre-treatment interaction to be dropped.

But what about the first pre-treatment period (1900 in your example)? Assuming a placebo treatment (1 for observations that are in treatment group and year is 1990), every observation in the treatment group is codes with one and then collinear with the individual fixed effects, leading the first pre-treatment interaction to be dropped.

But what about the first pre-treatment period (1900 in your example)? Assuming a placebo treatment (1 for observations that are in treatment group and year is 1900), every observation in the treatment group is codes with one and then collinear with the individual fixed effects, leading the first pre-treatment interaction to be dropped.