Whenever a container has some form of natural row it may align children in that row along a common typographical baseline. If the amount of vertical space in the row is taller than the total requested height of the baseline-aligned children then it can use a gtk-baseline-position to select where to put the baseline inside the extra available space.

(define-g-enum "GtkBaselinePosition" gtk-baseline-position
  (:export t
   :type-initializer "gtk_baseline_position_get_type")
  (:top 0)
  (:center 1)
  (:bottom 2))  

:top

Align the baseline at the top.

:center

Center the baseline.

:bottom

Align the baseline at the bottom.