  The solar cycle onset at mid-latitudes, the slow down of the sunspot drift
toward the equator, the tail-like attachment and the overlap of successive
cycles at the time of activity minimum are delicate issues in $\alpha\Omega$
dynamo wave and flux transport dynamo models. Very different parameter values
produce similar results, making it difficult to understand the origin of these
solar cycle properties. We use GONG helioseismic data to investigate the
progression of the solar cycle as observed in intermediate-degree global
$p$-mode frequency shifts at different latitudes and subsurface layers, from
the beginning of solar cycle 23 up to the maximum of the current solar cycle.
We also analyze those for high-degree modes in each hemisphere obtained through
the ring-diagram technique of local helioseismology. The analysis highlighted
differences in the progression of the cycle below 15\degr\ compared to higher
latitudes. While the cycle starts at mid-latitudes and then migrates
equatorward/poleward, the sunspot eruptions of the old cycle are still ongoing
below 15\degr\ latitude. This prolonged activity causes a delay in the cycle
onset and an overlap of successive cycles, whose extension differs in the two
hemispheres. Then the activity level rises faster reaching a maximum
characterized by a single peak structure compared to the double peak at higher
latitudes. Afterwards the descending phase shows up with a slower decay rate.
The latitudinal properties of the solar cycle progression highlighted in this
study provide useful constraints to discern among the multitude of solar dynamo
models.
